What would be the best ones to invest in if I want to make a drill only one as painless as possible?
Should I use alot of distraction based plasmids or whatnot?
Let me know if I understood correctly: You want to play through the game using just the drill and non-offensive plasmids? If that's the case, then Decoy should be the most useful one, since it distracts every enemy on the vicinity while you drill them to death or avoid them, but if you want to make it painless, don't upgrade it, since Decoy 2 inflicts damage to the enemies who attack it (although not as much as you'd expect).
Scout isn't offensive, but it isn't as useful as it could be, as it just allows you to go ahead into any area you'd like without getting noticed, since you play as Delta's "ghost" while you use the plasmid. However, if you upgrade it, it allows you to hack, should be useful even if you just aim for a drill only playthrough.
Security Command is a great distraction, but damages the enemy, more often than not killing him/her, so, you should avoid that, too.
Insect Swarm is also one of the best ones, especially against armored enemies, since it's basically harmless against them, but distracts them for a decent time.
Winter Blast doesn't inflict damage, but freezes enemies, you can take advantage of this, but you won't be able to loot them until you upgrade it and use the secondary attack.
Telekinesis allows you to shield yourself with any big enough object you grab with it as long as you keep holding LT slightly (holding it that way will keep the object in front of you, but if you completely hold it, the object will be at your left side and won't protect you from any damage), you can take advantage of this, drilling enemies without them doing anything harmful to you because of your improvised shield.
That's about it, really, the others (hypnotize, incinerate!, electro bolt, cyclone trap, etc.) are mostly offensive. I hope I was able to help!
well you dont get the drill specialist upgrade until late in the dionysis park level iirc, but i would go with some raneg based damage plasmids like incinerate and insect swarm while mixing it up with some tech plasmids like decoy and security evasion. also winter blast is good with the drill
So if death is nothing but a path to be trod, walk with me and let me introduce you to your god.