Easiest way to beat it is with peacekeeper since he seems to never counter it and has no way to deal with it (Dormant Sliver would be a nice addition and allow much faster kills - with Azorius Herald / Ink Leviathan for instance - but it doesn't work atm. Also he counters it when he can, usually).
Here is what I used, can be improved for sure but made it very easy :
4 Worldly Tutor
4 Peacekeeper
1 Survival of the Fittest
4 Veteran Explorer
4 Sakura-Tribe Elder
4 Prodigal Sorcerer
3 Dormant Sliver (useless, just fodder for Survival of the fittest)
4 Wrath of God
4 Winds of Rath
2 Aladdin's Ring
1 Ancestral Recall
1 Mystical Tutor
1 Black Lotus
4 Tropical Island
4 Savannah
2 Plains
1 Island
12 Forest
Couldn't find a faster way to kill with the peacekeeper out than Prodigal Sorcerer (he counters them sometimes), and Aladdin's Ring or Cursed Scrolls in case he didn't draw Harmonic Sliver. It's pretty boring to play though, because he keeps declaring attack phases when peacekeeper is out, lots of clicks and slow kills.
Edit : Made a funnier and more tweaked version with somewhat faster kills :
3 Plains
1 Island
1 Swamp
15 Forest
4 Savannah
4 Peacekeeper
4 Retribution of the Meek
1 Mystical Tutor
1 Demonic Tutor
4 Veteran Explorer
2 Algae Gharial
2 Sakura-Tribe Elder
2 Muscle Sliver
2 Survival of the Fittest
2 Cultivate
4 Worldly Tutor
2 Meddling Mage
1 Black Lotus
1 Mox Emerald
1 Mox Pearl
1 Mox Jet
1 Mox Ruby
1 Mox Sapphire
Get Peacekeeper out in first three turns, then tutor for 1 Meddling Mage (for Force of Will), 1 Algae Gharial, 1 Retribution of the Meek and Muscle Slivers if his slivers don't have 4+ power yet. Then 1-turn kill

(against Sliver Queen round, you might want to cast a first Retribution as soon as possible since I think the game might tend to crash if he makes too many slivers).