Blitzcrank 2XKO combo: 816 damage, expert+. The hardest part of this combo is the tag, you should do it as late as possible, as shown in the video :)
M > H > 2 + H > 9 > j.M > 2 + j.H > j.H > 2 + H > 9 > 2 + j.H > M > H > 2 + H > 9 > j.H > j.S1 > T > T > |2 + H > 9 > j.M > j.H > j.S1 > S2 + L > S2 + L
The hardest part of this combo is the tag, you should do it as late as possible, as shown in the video :)