Considering that you are more defensive rather than offensive, then getting to an out of shield grab, trap, or jump cancel Up B (at higher percents) will be most of the tools you will use. Here I'll cover a couple of things and what you could do with them based on your playstyle.
walk/run cancel shield:
Just because you are defensive does not mean you should stay still against your opponent. That said, you should move around and cancel your movement with the shield when you feel threatened by your opponent. This not only keeps them from dominating the pace of the fight, but it also adds pressure that a defensive player should display for being there to punish mistakes.From what I saw, you already know how to do this, so I won't recap on it (otherwise, you can check on my guide under the Dash cancel shield in the basics section for more details). Anyways, you want to condition your opponent to think that he could try to attack you, but you block it and go into something else. Practicing to perfect/power shield too is key towards gaining the upper hand against aggro opponents, especially fast characters and projectile users. Projectile users especially will force you to agro them more than the other way around, but safely poke using the FAir assault tactic and NAir, and it should keep you safe for the most part. You should also use this more than your standard running grab if you can help it, as it is much safer than the latter.
Out of shield options:
As you already know, these options will be your most utilized tools in a defensive playstyle. Your grabs, especially your up throw, will be important towards racking up the damage and eventually getting the kills. If your opponent approaches in the air, more times than not you will be able to shield whatever they throw at you and grab them. Another option available to you is your shield cancel jump, which allows immediate retaliation no matter the the blockstun. As soon as the attack seems safe to retaliate from, quickly SH/FH and attack with either the FAir or NAir. This jump cancel also has the function of going into your Up Smash, which is much more useful in Lucina's hands than Marth's. Essentially, as you press the jump button, immediately perform the Up Smash attack, and it will take priority over actually jumping. So this can be used to counter opponents that land behind you. Once around 60% or more, you'll have the option to combine the jump cancel with your dolphin slash, which is an incredibly fast and high priority move. It is not invincible by any means, but it can be a great way to retailiate and get your opponent off stage (though you may be better off with a grab more than anything else.
When not in shield:
Not all opponents will simply keep attacking for you to keep countering them as you know; they will likely go for grab mixups or even moves that are safe to poke with and keep chipping at your shield. It is at this point where you will need to outspace them with Lucina's normals. Starting with FTilt and UTilt, these two options are used to catch your opponents as they jump towards or above you. You want to keep them at a sword's length away at all times to keep the defensive stance. Your Dtilt is used to safely poke at your opponent if they land infront of you and/or try to enter a shield state as well. You want to get a reaction using the Dtilt, otherwise keep poking till they try to spot dodge or roll to either side, then turn around FTilt/FAir/DB them as they complete it. Jab is also another great option for this, and can be used as an anti air setup into another attack (will be writing on this on my guide right now actually, may finish by tonight or tomorrow xD). Safely poke, but keep track of your shield's damage at all times. These not in shield options is to keep your opponents off and give time for your shield to recover, so use them as you feel is appropriate.
Pivot Ftilts/FSmashes/grabs:
Needless to say, there will be moments where you may need to retreat to keep your opponent from getting too close to you. If ever you feel threatened by a running up grab or attack, run backwards and try for one of these options to keep you covered. As soon as they see any of these, most characters will back off a bit, giving you a bit of time to breathe and focus your thoughts on your next attack. Never let your opponent dictate the pace, as speed characters especially want this in order to punish you. Another important note to make is that FTilt and FSmashes can be risky against fast characters, as the commitment if read correctly can lead to you being combo punished. So your best option is the pivot grab outside of the above options.
Going on the attack:
Playing defensive won't always get you the stock, so you'll also need to learn to attack them when you get your chance. Stick with the fundamental NAir or FAir assault, like mentioned earlier, to press your advantage forward and get them off stage. From here, follow the advice I gave earlier for your offstage tactics and keep racking the damage like so. If you can get the read, go for your SB or FSmash, but don't use them often unless you see a good opportunity to hit them.
Primary objective:
Here's the important part for all of this I've put out for ya; you're not looking to kill your opponent as quickly as possible. A defensive game will pick at their opponents thoughts and keep walling out their options. As soon as a point of weakness shows up, you're ready to finish them off with a gimp or Smash. If it means that you'll keep racking up the damage to extremely high percents (like 150%), don't feel like you're in a hurry to finish them off. Keep yourself safe as best as you can, and if ever they go to that percent, you'll be able to up throw them to finish them off right there. Take your time, and the results will pay off in the long run.
