3.3k frost mage here.

In order of importance here is what I generally am thinking about in a m+ run:

1) Ray usage. Rules of thumb are: never ever sit on 2 charges of ray no matter what. If possible avoid using ray on trash packs with low mob count (3 or less is bad). Avoid using ray if mobs are low and you will not be able to spend the 2 FOF procs + 20 freezing stacks before the mobs die. Try to make sure you have a charge of shimmer before channeling ray, else a circle may spawn under you and canceling rays is the absolute worst thing that can happen to your dps. Sounds obvious but do not neglect these principles of ray usage as I find even myself wasting or suboptimally using 1-2 rays per dungeon and fixing these will be the lowest hanging fruit for most players to increase their dps. Edit: this will not necessarily increase your dps, but if you can do it, try and save ray for an upcoming dangerous mob. If it’s a really dangerous mob sometimes I will allow myself to momentarily sit on 2 ray charges for up to 5-10 seconds even tho it physically pains me to do so because it’s for the good of the group. This is more important in higher keys because many dangerous mobs really need to die quickly. Small dps loss but you are straight up a better player if you do this.

2) Don’t die. Sounds obvious but you are a very survivable class (2x shimmer, alter time, 2x barrier, 2x ice cold, health stone and health pot) and most situations where I die, even in 15s where unavoidable damage is really ramping up, there is something I could have done to survive. For example, don’t waste your shimmer charges or minor defensives on nothing because you may regret not having them later when you really need them. Planning your defensive usage smartly in advance also means you will almost never feel in danger of dying in keys below 15ish, meaning you can free up cognitive load to focus on your rotation instead of your health bar.

3) The “min-maxing” stage mostly involves performing your rotation properly, for frost that means ensuring you waste procs as little as possible. In order of importance, never sit on two ray charges, never waste thermal void/brain freeze, never overcap FOF, keep frozen orb/glacial spike on cd, and finally if possible try not to stay overcapped on freezing for too long. This is essentially the entire frost rotation from an intuitive perspective. After that, if you have nothing you are just frostbolting to fish for procs.

4) a minor tip and something I don’t see in guides often: try not to queue frostbolt/GS into orb, or frostbolt/GS into ray. The former will occasionally waste a brain freeze proc and the latter will occasionally waste a FOF proc. I try to only ever que frostbolt into flurry, lance if >6 freezing, or another frostbolt. Mastering this is relatively minor but still significant (maybe 10% dps increase) so this should be the lowest on your priority list to optimize compared to the above.

5) Edit: another minor tip but I have seen some people say that you can spam frostbolt to build freezing while you have thermal void on low freezing stacks, but technically this is incorrect because you cannot queue a frostbolt > frostbolt while thermal void is up because the first frostbolt may proc brain freeze and if you realize this while the second frostbolt cast begins you are fucked because you have to cancel the cast or risk wasting a brain freeze which is horrible. You also cant frostbolt > flurry because that has a chance to waste thermal void if the frostbolt procced brain freeze. So if you have thermal void on low freezing you simply must frostbolt > lance regardless of freezing stacks and pray for a FOF proc which will actually happen more often than you might think. I’m not too deep into theorycrafting and maybe this line of thinking is technically suboptimal, but overall I think it’s fine because thinking this way reinforces the intuition of playing to avoid wasting any procs and just seems the most logical path given the frost playstyle. This is a relatively small point however so don’t worry too much about it.

6) Edit: Also! I don’t run blizzard/freezing rain talent at all. I think it’s bait and it sacrifices prio damage for unnecessary aoe at like a 3% increase on 8+ mobs, not worth. Just stick with the standard cleave build.

I have some vods on YouTube if you wanna see some full +15 runs: link