These are some moderately simple things which will allow you to spam PK Fire more safely and faster.
Short Hop PK Fire
More important to talk about than you might at first think. PK Fire has less lag on the end depending on how far away you are from the ground when you use it. This varies even by small amounts. Using PK Fire just before you land will have significantly less lag than simply short hopping and immediately using PK Fire. Practice the timing so that PK Fire comes out as low as possible but DOES actually come out. Not just Lucas screaming. For B-stickers this means you want to B-stick your PK Fires slightly later than usual but it's not much because the wavebounce effect pushes you downward naturally. For non-B-stickers you'll have to work out the timing to use it just before you land.
Lagless Full Hop PK Fire
A very useful thing to know. Do a full hop and IMMEDIATELY PK Fire and you'll be able to do your second jump before touching the ground. This means there is literally zero landing lag because well you're not landing. This provides a lot more safety using full hopped PK Fires because it allows you to move out of the way after you've used it in case you missed. It can work as a nice retreat if you use it running in the opposite direction as well. Even better it can allow you to follow up with another PK Fire directly after the first at the same level which can be very handy if an opponent is on a platform above you or is trying to approach from the air. The timing is tricky so you'll have to practice this one. It's close to the timing of multishining in Melee but not quite.
For B-stickers the timing is a lot easier and it works brilliantly as a retreat since both wavebounces will push you backward.
Utilizing Platforms and Stages
As mentioned earlier, the closer you use PK Fire to the ground the less lag you have on the end. Some platforms can really help you out here as they give you a handy place to land. Jumping off stages and preparing PK Fire early so that you land shortly after starting it can reduce lag a lot as well.
The fastest repitition of PK Fire possible is done this way. On Battlefield go to the top platform. Run off one of the sides and immediately use PK Fire toward the center of the stage. PK Fire will push you toward the edge of the platform. Quickly press toward the edge of the platform to fall off almost immediately and then quickly PK Fire again toward the center of the stage. If you time it early enough you will go off the stage. Use your double jump and PK Fire as your approaching the stage for a third very low lag PK Fire. This is all w/o B-stick. It's epic.
Ground PK Fires
In most cases you're better off jumping before you PK Fire. This is because there is less start up time when you use PK Fire in the air. However every once in a while, using one on the ground can work to your advantage. WHENEVER YOU USE PK FIRE ON THE GROUND TILT THE ANALOG STICK, DON'T SMASH IT. This will prevent you from taking the risk of tripping that occurs any time you smash the analog stick.
While dashing in one direction you can immediately stop and use PK Fire in the opposite direction by pressing forward B in the direction you're facing and quickly moving the analog stick in the opposite direction for the turn around. It takes some time to learn but you can catch people off guard with it sometimes so it can be useful.
Short Hop PK Fire
More important to talk about than you might at first think. PK Fire has less lag on the end depending on how far away you are from the ground when you use it. This varies even by small amounts. Using PK Fire just before you land will have significantly less lag than simply short hopping and immediately using PK Fire. Practice the timing so that PK Fire comes out as low as possible but DOES actually come out. Not just Lucas screaming. For B-stickers this means you want to B-stick your PK Fires slightly later than usual but it's not much because the wavebounce effect pushes you downward naturally. For non-B-stickers you'll have to work out the timing to use it just before you land.
Lagless Full Hop PK Fire
A very useful thing to know. Do a full hop and IMMEDIATELY PK Fire and you'll be able to do your second jump before touching the ground. This means there is literally zero landing lag because well you're not landing. This provides a lot more safety using full hopped PK Fires because it allows you to move out of the way after you've used it in case you missed. It can work as a nice retreat if you use it running in the opposite direction as well. Even better it can allow you to follow up with another PK Fire directly after the first at the same level which can be very handy if an opponent is on a platform above you or is trying to approach from the air. The timing is tricky so you'll have to practice this one. It's close to the timing of multishining in Melee but not quite.
For B-stickers the timing is a lot easier and it works brilliantly as a retreat since both wavebounces will push you backward.
Utilizing Platforms and Stages
As mentioned earlier, the closer you use PK Fire to the ground the less lag you have on the end. Some platforms can really help you out here as they give you a handy place to land. Jumping off stages and preparing PK Fire early so that you land shortly after starting it can reduce lag a lot as well.
The fastest repitition of PK Fire possible is done this way. On Battlefield go to the top platform. Run off one of the sides and immediately use PK Fire toward the center of the stage. PK Fire will push you toward the edge of the platform. Quickly press toward the edge of the platform to fall off almost immediately and then quickly PK Fire again toward the center of the stage. If you time it early enough you will go off the stage. Use your double jump and PK Fire as your approaching the stage for a third very low lag PK Fire. This is all w/o B-stick. It's epic.
Ground PK Fires
In most cases you're better off jumping before you PK Fire. This is because there is less start up time when you use PK Fire in the air. However every once in a while, using one on the ground can work to your advantage. WHENEVER YOU USE PK FIRE ON THE GROUND TILT THE ANALOG STICK, DON'T SMASH IT. This will prevent you from taking the risk of tripping that occurs any time you smash the analog stick.
While dashing in one direction you can immediately stop and use PK Fire in the opposite direction by pressing forward B in the direction you're facing and quickly moving the analog stick in the opposite direction for the turn around. It takes some time to learn but you can catch people off guard with it sometimes so it can be useful.