Frost and Cha Ye Trinket
For most people though, it's Crit -> Haste. Especially because you already have Wushoolay, which will benefit from added Haste.
If you follow AMR's gearing guide, it will probably ask you to slot Cha-Ye and Wushoolay for normal tier 15. If you switch to heroic tier 15, it will probably still suggest Wushoolay and Cha-Ye. The reason being is that Frost can get an extremely good proc rate on Cha-Ye because it doesn't have an ICD and it's proc rate is [0.8085*(1+SpellCritChance)]. Shatter kicking in with FFB and IL produces crits at a reliable rate, helping its proc.
Hydra offers a 20 second int boost, but it doesn't allow Frost Bomb play and relies on a high Haste to keep up. It's got a 0.525 RealPPM on damage/absorb from a periodic spell. It will almost always restrict Frost to Nether Tempest because of how fast it ticks. But a high haste and Nether Tempest usually results in DPS losses in other ways. On single target DPS, Living Bomb beats Nether Tempest. Frost Bomb also offers more DPET than Nether Tempest on single target and NT only shines on 2-5 target fights, DPS-wise. NT will also yield Brain Freeze proc overrides and losses as you pile on more Haste (especially when other Haste procs like your Legendary Meta & Time Warp go off).
AMR isn't as reliable for high end raiding mages but it's a good starting point. Cha-Ye is actually a good trinket if your Haste is still relatively low. There are guides posted over the internet which suggest that Frost only stack Haste to 23% or so (with buffs). Most people go around asking everyone to gear Frost like this when they don't take into account two things:
1. The guides will only recommend that for a high ilvl, which you don't have yet.
2. To make 23% Crit work, you need a lot of haste and 2 RPPM trinkets that gave int. Meaning your high proc rates will almost always put you at or around 28% Crit anyway. The guide doesn't ignore the soft Crit cap, it assumes that once you have enough Haste, you will be there more often than not.
Which I feel is the problem with guides, they're generic and can't possibly cater to everyone unless the entire community is actively progressing in raids with a high ilvl, which isn't the case.
Cha-Ye is your only valid second trinket choice if you run Frost Bomb. If you don't raid, Cha-Ye and Wushoolay are your best trinkets, because of how powerful Frost Bomb is for solo PvE, scenarios, questing, and such.
Hydra is a great trinket for Frost if you play with NT/LB all the time (but Frost Bomb still wins on field solo and other aspects where you can freeze your target and will never proc Hydra).
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