65 hammer, 70 hammer, 70 relic, 70 staff (kinda sorta optional based on play style).
An orange sleep hammer is optional. Honestly when I played 70 Mystic, I had a orange sleep hammer but very... very rarely used it. If at all.
Mystic is very gear dependent. You're probably gonna win most of your battles in arena by either out gearing, playing your class advantage, or sheer RNG luck with your procs on weapons and damage. Really just how the class goes.
Mystic VS DS is really about stun landing/hit rate > damage. If you want, throw on a pontiff hammer. Kite them with range KB, root, and AoE stun + bird if you really have to. I never really found DSes too problematic when I played Mystic, so I mostly just stacked hit rate and derped in wolf. But builds have changed a bit since I played~
Mystic VS SS is probably gonna be your toughest point. If an SS gets a jump on you, just pray sleep procs, root, get close, heal up with HoTs, stun if he doesn't have mongoose, use the AoE -hit Ape debuff, and hope for the best. Your best chance of surviving an SS is really your sleep procs, how much damage your build can take, how well you can stack your HoTs.
If however you do get the chance to jump them, wait until mongoose is over, use pontiff here and there if your damage is low on them, and hope you can stun lock before they can weasel back to shoot you to death.