Here's a tutorial I have put together to defeat the king boss~
Ok, to start off-
1. Hide behind the wall northwest of you when you spawn after the cutscene
2. Shoot the octorocks and gomas surrounding him to weaken his defenses
3. Look in the garbage can by the archway to the right of you for the EMP rifle
4. Shoot him with it. This will stun him
5. Now's the time to use the enclosed instruction book- aim for his head! Repeat step 2, 4 and 5 until he dies
6. Victory!
Not hard at all, really. Just watch out for his dinner blaster. If not, you'll end up like mothra in my GIF avatar
Hope this helps.
I found a glitch!!!
if you use the homing RPC and aim at the chandelier above him, it'll glitch him out of the game. now just run up to where he was standing and you'll go to the winning cutscene
another edit: if you have the flame upgrade for your hRPC it will just catch him on fire. any other upgrade works for this glitch