Lucas's Dair is an amazing move. If he didn't have it he would probably be around the level of Captain Falcon. It allows spacing, juggling defense, a high priority aerial, an approach and most importantly, deadly combos. But comboing with Dair effectively is more complicated than you might guess. I've spent a lot of time on it and probably contradicted myself atleast twenty times along the way. But I believe the following will provide a helpful guide to the most effective uses of Dair.
The Pitfalls of Dair
First, understand that Dair is not infallible. Although it has high priority below Lucas, it has very little horizontally. So if they know to hit your side, they can stop it everytime. Also, if the opponent DI's upward, they can often escape before getting hit by the fourth "spike" hit. Also, although it is uncommon, Dair can be teched making combos much more difficult. More on that later. Finally, it's very easy to overuse Dair causing it to go stale and not be as effective.
Dair as an Approach
Dair is useful as an approach because it is very difficult to punish. First of all, you can space with Dair by full hopping toward the opponent and retreating. This will keep you from getting too close should they shield and you can avoid being punished. Also, against a shield Dair has a disadvantage of 9 frames assuming you autocancel. This is remarkably low compared to a lot of moves in this game. As long as you space yourself properly in front of your opponent, or go behind your opponent with Dair, they will be unable to shield grab or do any other move out of their shield to punish you. It's quite effective.
Low Percentage Dair Combo from Approach
Congratulations! Your opponent just walked into your well spaced Dair. What are you going to do now? Well if you're smart, as soon as your Dair connected you realized you should move behind your opponent so you can't be shield grabbed. Also know that you MUST FASTFALL AND AUTOCANCEL TO COMBO!!! Immediately following you have several options that will vary depending your opponent. The most effective moves to combo into after Dair at low percentages are jab, Ftilt and Nair. All of these are meant to rack up damage. They also come out very fast. All of them should work most of the time. HOWEVER, Nair and Ftilt are not always effective. Characters with moves that come out behind them quickly will be able to stop you. This includes ROB, Wolf, MK and probably a few others. At very low percentages you will HAVE to jab or else they'll hit through your Nair and be able to attack before Ftilt comes out. Jab comes out in 2 frames so it will pretty much always be effective. As soon as you rack them up to a bit higher percentages however, you will be able to use Nair or Ftilt.
Some other options that are not preferable and why...
Dtilt - players can shield after the first one and punish
SH Uair - too slow at low percentages and can be punished due to cooldown
SH Fair - too slow at low percentages and can be punished due to cooldown
Utilt - can be punished due to cooldown
High Percentage Dair Combo from Approach
At around 80% or so (depending on the character,) your opponent is in Dair combo kill range. If you hit with atleast hits 3 and 4, your opponent will be knocked on their back leaving you many options. The most effective kill combo from here will vary depending on the situation.
Not near and facing an edge...
- Dtilt lock as needed (it shouldn't take much or at all,) walk forward slightly and Usmash
Near an edge facing outward of stage...
- Fsmash immediately, Dtilt can knock them off especially if they DI away.
On a stage with ceilings (Luigi's Mansion), not near an edge...
-Dtilt lock out of range of ceiling and Dsmash or Usmash (this will depend on character your Dtilt locking and their DI to avoid knocking them off the ledge. Dsmash if unable to completely clear ceiling. Make sure you wait and hit with the first hit of Dsmash rather than the second or third for best kill potential.)
If your opponent knows how to tech Dair...
- Fastfall and immediately hyphen smash tech chase. It's really hard but it gives you a free Usmash and there's nothing else you can do really.
At the same percentage, if you hit with only the 4th hit, which is common against shorter characters and annoying, you have less reliable options. It is difficult to see for sure when this is going to happen and you pretty much have to predict it. The opponent will be sent slightly upward. They will usually airdodge immediately. To punish, Dsmash immediately. If you don't predict it and it catches you offguard, you can often shield and then shieldgrab in their landing lag or punish some stupid aerial they tried out of surprise. Avoid this if you can.
Dair as Punishment Out of Shield or From Close Range
When you're very near an opponent, you can Dair immediately from a jump to pull them upward and slam them back down. Unfortunately, you can only truly combo with this at high percentages. However, it is still very effective for getting off kill moves. Fastfall immediately and Dtilt. YOU MUST DTILT FOR ANY KIND OF COMBO IN THIS SITUATION BECAUSE YOU WERE NOT ABLE TO SPACE YOUR DAIR VERTICALLY AS WELL AS YOU CAN IN A REGULAR APPROACH. From here you can Dtilt lock, Dsmash or Usmash. It will follow the same rules as listed for high percentage Dair combos above.
Summary
Dair combos kick ***. They let Lucas actually get some kills outside of gimping. But you must know when to use which combo. And you must learn how to space your full hop Dairs. I hope this gives some of you some ideas on the most effective combos. I think I covered about every situation. Some of these are more difficult to do than other combos but have greater reward and are very possible. Took me a while to get out of the habit of doing the easy Fsmash after Dair, to spacing approaches so I can always Dtilt for earlier Usmash kills.
The Pitfalls of Dair
First, understand that Dair is not infallible. Although it has high priority below Lucas, it has very little horizontally. So if they know to hit your side, they can stop it everytime. Also, if the opponent DI's upward, they can often escape before getting hit by the fourth "spike" hit. Also, although it is uncommon, Dair can be teched making combos much more difficult. More on that later. Finally, it's very easy to overuse Dair causing it to go stale and not be as effective.
Dair as an Approach
Dair is useful as an approach because it is very difficult to punish. First of all, you can space with Dair by full hopping toward the opponent and retreating. This will keep you from getting too close should they shield and you can avoid being punished. Also, against a shield Dair has a disadvantage of 9 frames assuming you autocancel. This is remarkably low compared to a lot of moves in this game. As long as you space yourself properly in front of your opponent, or go behind your opponent with Dair, they will be unable to shield grab or do any other move out of their shield to punish you. It's quite effective.
Low Percentage Dair Combo from Approach
Congratulations! Your opponent just walked into your well spaced Dair. What are you going to do now? Well if you're smart, as soon as your Dair connected you realized you should move behind your opponent so you can't be shield grabbed. Also know that you MUST FASTFALL AND AUTOCANCEL TO COMBO!!! Immediately following you have several options that will vary depending your opponent. The most effective moves to combo into after Dair at low percentages are jab, Ftilt and Nair. All of these are meant to rack up damage. They also come out very fast. All of them should work most of the time. HOWEVER, Nair and Ftilt are not always effective. Characters with moves that come out behind them quickly will be able to stop you. This includes ROB, Wolf, MK and probably a few others. At very low percentages you will HAVE to jab or else they'll hit through your Nair and be able to attack before Ftilt comes out. Jab comes out in 2 frames so it will pretty much always be effective. As soon as you rack them up to a bit higher percentages however, you will be able to use Nair or Ftilt.
Some other options that are not preferable and why...
Dtilt - players can shield after the first one and punish
SH Uair - too slow at low percentages and can be punished due to cooldown
SH Fair - too slow at low percentages and can be punished due to cooldown
Utilt - can be punished due to cooldown
High Percentage Dair Combo from Approach
At around 80% or so (depending on the character,) your opponent is in Dair combo kill range. If you hit with atleast hits 3 and 4, your opponent will be knocked on their back leaving you many options. The most effective kill combo from here will vary depending on the situation.
Not near and facing an edge...
- Dtilt lock as needed (it shouldn't take much or at all,) walk forward slightly and Usmash
Near an edge facing outward of stage...
- Fsmash immediately, Dtilt can knock them off especially if they DI away.
On a stage with ceilings (Luigi's Mansion), not near an edge...
-Dtilt lock out of range of ceiling and Dsmash or Usmash (this will depend on character your Dtilt locking and their DI to avoid knocking them off the ledge. Dsmash if unable to completely clear ceiling. Make sure you wait and hit with the first hit of Dsmash rather than the second or third for best kill potential.)
If your opponent knows how to tech Dair...
- Fastfall and immediately hyphen smash tech chase. It's really hard but it gives you a free Usmash and there's nothing else you can do really.
At the same percentage, if you hit with only the 4th hit, which is common against shorter characters and annoying, you have less reliable options. It is difficult to see for sure when this is going to happen and you pretty much have to predict it. The opponent will be sent slightly upward. They will usually airdodge immediately. To punish, Dsmash immediately. If you don't predict it and it catches you offguard, you can often shield and then shieldgrab in their landing lag or punish some stupid aerial they tried out of surprise. Avoid this if you can.
Dair as Punishment Out of Shield or From Close Range
When you're very near an opponent, you can Dair immediately from a jump to pull them upward and slam them back down. Unfortunately, you can only truly combo with this at high percentages. However, it is still very effective for getting off kill moves. Fastfall immediately and Dtilt. YOU MUST DTILT FOR ANY KIND OF COMBO IN THIS SITUATION BECAUSE YOU WERE NOT ABLE TO SPACE YOUR DAIR VERTICALLY AS WELL AS YOU CAN IN A REGULAR APPROACH. From here you can Dtilt lock, Dsmash or Usmash. It will follow the same rules as listed for high percentage Dair combos above.
Summary
Dair combos kick ***. They let Lucas actually get some kills outside of gimping. But you must know when to use which combo. And you must learn how to space your full hop Dairs. I hope this gives some of you some ideas on the most effective combos. I think I covered about every situation. Some of these are more difficult to do than other combos but have greater reward and are very possible. Took me a while to get out of the habit of doing the easy Fsmash after Dair, to spacing approaches so I can always Dtilt for earlier Usmash kills.