Invincible? Unless you're relegating yourself to wooden weapons, I doubt they take more than a few seconds to kill. Also, I've never seen a zombie/skeleton with obsidian armour, and they never seem to come with more than two bits of armour at a time. A minor pain in the ass early-game, but after about two hours, I have an iron sword, and can deal with them handily. Unless you're playing on Hard difficulty, or something, I suppose...