I don't know if anyone's already mentioned this (didn't want to read through all 4 pages of responses) but when you're fighting Pig Josh in Red Dead Revolver, there's a building with a broken roof you can jump on that where he can't get you with his explosives. I did that the first time playing through.
Also, in DeathSpank, I horded all my invincibility orbs and used them during tough boss fights, leaving me free to simply hack and slash my way to victory without downing a single potion.
Edit: Also, not really a boss, but in Morrowind I really wanted some Ordinator armor, so I got a bunch of money, went to Vivick, and attacked the first one I saw that was alone. When he turned and told me to pay a fine or go to jail, I paid the fine. Then I hit him again. I just repeated this process until he died and grabbed all his shit.