Combos Utilizing King-Crab

Have Furnace of Rath in play. If opponent has green creature out, tap King Crab & put green creature on top of opponent's library. Cast and sac Booby trap to have it do 20 damage to your opponent.

spend 1u and tap king crab to put opponent's green creature on top of their library. cast ray of erasure to put that creature into opponent's graveyard.

sepnd 1u & tap king crab to put target opponent's green creature on top of his library. cast predict & name that creature. it goes to opponent's graveyard & you draw two cards.

put the skyshroud behemoth into play then when it would die just put it back on top of your library

Have King Crab in play, plus an Elvish Piper if you wanna make things easier, then put in deranged hermit, put him back on your library, and play him again. Each time attack while the hermit is in.