Farming The Warrior end-game boss in Borderlands 2 with the Bee shield + Conference Call shotgun. He's dead in seconds. Even if you don't have those, you can stand in a safe zone where none of his attacks land and pick him off. I mean, screw it, it's a game that makes you farm to get good weapons.
Killing the Bunker boss in Borderlands 2 by hiding under a large overhang. He can't hit you there because he's a giant-ass ship flying around above you.
Killing Rhinos in Far Cry 4 by standing on a rock. Seriously, to hell with those guys, they suck and they have zero vertical leaping ability.
Edit: Oh yeah, this just happened a couple days ago in Divinity: Original Sin. There is this giant spider boss that I went around. I could see her but I didn't feel high level enough to fight her so I circled around and was going to do something else until I leveled up. However, there is like a ledge behind her and I wondered if starting the fight from there would make it easier so I saved and tried it. Turns out she can't reach you there with melee OR poison attacks. I was completely safe. My attacks wouldn't reach her, either, BUT I could summon monsters down where she was standing to attack. She killed them pretty fast but I have three characters who can summon so I just did that until she was dead. Cheap as hell but I love killing monsters that are higher level than I am.
