I just remembered someother good ones:
The Fear, if you use the tranquilizer gun, he'll die extreamly quickly. Every time he jumps, he uses a lot of stamina, so that makes him pass out even faster. Also, when he goes to get food, just stab a bunch of poison dart frogs. He'll always eat something that's already dead, so he'll eat all the poison making him even weaker. This is a lot harder to do if you're playing on hard or any higher difficulty, but it's extreamly to do on normal.
Starman Jr., you actually have to fight him, but you can't take any damage during the battle. Buzz Buzz will always create a shield, making it impossible to die, so all you have to do is use attack over and over.
The first boss battle in Sonic the Hedgehog 2 was unbelievably easy. All Eggman will do is slowly move across the screen, so just jump on him. The only hazard is the spike on the front of his car, but it's nearly impossible to get hit by it.
Pretty much every boss battle in Resident Evil 4 can be killed in one hit, all you have to do is use the rocket launcher. Obviously this takes away all the fun, so don't use it.