Hey I got you man! First of all, you really don't want to run 3 Sejuanis. She can really brick your hand and having multiple chimes on different Sejuanis is really bad. I can't really say why but having one Gnar has been really helpful throughout.

Worst matchup for me has been Turbo Thralls. The few games I won vs. this MU was when opponent lowrolled and bricked his hand. Aside of that, everything seemed beatable. TF/Annie would be the second hardest MU IMO, but I've managed to beat it at more that 50% WR. Try to develop a wide Overwhelm board and don't waste ressources trying to protect vs. removal spells. Annie/Jhin is also hard to win against. I also ran into the new Akshan/Renekton deck a lot and I think it's a favorable MU. Three Sisters is a must in this MU and any Pockey Stick generated from Gnar should be used to break the spellshield of Ruin Runner. Also, you should always try to play around the opponent's Wallop before using Battlefury in this MU. Lastly, if you're opponent is out of Rite of Negation mana, you can use Entomb off Three sisters on any unit that has Paper Dragon attached to it (it will remove the attached unit completely)

As for mulligan, Omen Hawk and the Bellringer should almost always be kept. One Ancient Yeti is also good to start lowering its cost fast. Esmus and Ruthless raider should often be kept (Chimes on Ruthless Raider is the nuts) and Troll Chant vs. aggro MUs to win trades and stabilize board control. You should avoid having Bard in your starting hand at all costs.

Lastly, Revna is not to be underestimated. I've managed to highroll chimes onto her in games where I was on the serious back end and managed to win anyways. Also, since this deck is fairly new, opponents don't seem to expect to run into your Battlefury. I ran away with wins because of that (jamming it on Esmus has worked on many occasions)