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Kassadin Multistriker
This is a comp you want to reroll at lvl 6 with item priority on Kassadin and Akali. This comp requires you to have a lot of items so pick augments like Living Forge, Buried Treasures or Pandora items or play it with Portals such as Loaded Carousel.
Once you have your 3 stars, push your levels and play more and more Multistrikers because they give a lot of power to Akali and Kassadin.
Syndra Shapeshifter
This is the most consistent way to play Syndra, there are a few variations (like 7 Eldritch, or with 3 Mages and 4 Vanguards).
Make sure to roll at 6, stack your Syndra as early as possible and you'll have a good game.
Kogmaw Reroll
This comp requires many 3-star units and you can stay level for a long time and it's fine. You can even 3-star Jinx if you have stuff for her.
Roll at level 6 until you find 3 star Kogmaw and at least one tank at 3 stars.
Blitz is the best tank, but if you can't make it 3 stars, aim for Nunu and Rumble as your best tanks.
Karma Warrior
This is a very expensive comp and I suggest you play only if you managed to snowball or if you have an insane amount of gold so you can rush your lvl 8, stabilize with your 4 cost units (Karma, Fiora and Gwen) then save your gold and push lvl 9. A perfect example is, for instance, if you managed to snowball with an econ augment at 2-1.
Kalista Preserver
And finally, this is another S+ comp but also requires a lot of gold and requires you to snowball your early and mid-game. This time you will rely on Kalista as your main carry and 5 cost units as side carries (Morgana, Camille, Diana).
You comp will be incredibly hard to kill if your opponent doesn't have anti-heal!
And that's it for the top 5 Most broken comps you want to play at the start of Set 12!
I made a cheat sheet about the best artifact holders, make sure to bookmark it!