So I am starting a home game soon, and considering one of my potential big bads later on to be a warlock. I like the idea that they can do some odd stuff, and blast all day sort of like a kineticist. Is there anything I need to do to keep it in lieu of a pathfinder capable character? Nothing is worse than a badly designed boss fight.
I'm building him currently to be more or a harasser than an outright drag down fight. He is currently capable of spiderclimb, summon swarm and darkness invocations, so he can creep around on the ceiling and just send swarms at the group when they are already occupied, with the opportunity to reveal himself and blast when it would probably take someone down.
You're the DM. One thing that I've worked on both in my games and helping to institute in games that I play in, is more description and visualization of what's going on, especially with spells. So I don't just say "he casts Fireball," but I say "Those close to the wizard see a small glowing marble fly from his outstretched finger, and you have enough time to say 'oh ****' before it explodes over your head." It creates more drama I think, but it also then allows you to have your NPCs do actions that may not be 100% pulled from rulebooks but sounds like something that could exist in the world you created. I know this isn't exactly what you are asking, but as long as you think this baddie will create an interesting fight for the party, it's all good.
If it's an NPC then a sorcerer would be more flexible in representing the warlock's mechanics. No awkward mechanics like burn to deal with which a warlock just doesn't have, and a wider variety of spells you could choose for them. Blasting all day isn't something an NPC on the other side is going to have a chance to do. Ever.
If you're just using warlock mechanics rather than choosing a PF class to represent them never mind that. Though the warlock might make a less than impressive BBEG, it might be worth dropping them a CR or 2.
Use the kineticist as a template for blasting damage, give them the spell like abilities you want the character to have instead of utilities.
Don't use an element, instead just describe the blast as green eldritch energy or however you imagine it.
For the harasser aspect you can use the wall infusion and ride the blast. Won't have those both until level 12 but if that's too high you can always just let the NPC have them without making them that high a level.