Im most inclined to suggest a Voltron playstyle for Kefka, since both halves benefit from combat buffs, but honestly, you can build any deck around him
Usually led by Marrow-Gnawer or Karumonix, the Rat King, they break the singleton rules of the format, make players groan while they resolve Thrumming Stone trigger after Thrumming Stone trigger, and generally go aggro
By opening packs, of course
Still, you may be tempted to use the other options depending on the MTG format youre playing
If a [Rush: Character] is played and attacks a character on that turn, can OP14-060 Donquixote Doflamingo's [On Your Opponent's Attack] effect be used to change the target of the attack to your Leader Donquixote Doflamingo
= Check out the coverage of Pro Tour - Venice at Sideboard.com (starting today!) to see if these are played in Onslaught Block Constructed