Upon spotting you, Skellobits will charge at you, attempting to skewer you with their spears. Jumping on their heads is effective against them, except for the ones with spikes on their heads then it's effective against you. All other attacks are good against them(except those that do no damage such as Mario's F.L.U.D.D.).
There are also flying Skellobombers who pull off and throw their own heads(don't worry, the heads grow back). And green, worm-like Skellobaits who attack with icy breath.