This is what I usually do in my ET rotation for reference
Always start off with Tri Disaster Charm if you are going to stay at one spot for long time. Some examples would be ET mob killing stages or CMs. In dungeons it might not be necessary because you DPS might be strong enough without it
In ET my main goal is not really to deal a lot of damage (even though I can) but more to CC mobs.
Frost Pillar would be the best skill to start off. It holds mobs for a pretty long duration. I always spam 3 Ice Blast during my Frost Pillar rotation, with enough time to even spam 3 Eradication after that.
Next I would use Magnetic Force to group up mobs and Heavy Gravity. During that I would use my Ice Pike to make sure they stay in that area.
After that, I would use Gravity Pole to hold mobs. If I didn’t use Magnetic Force earlier I would use it here.
Finally, I use Snow Rolling to gather mobs again. By the time this ends, your Frost Pillar CD should be almost up and you can repeat again.
I play this “Ice Mage” as a sub for fun, mainly to CC mobs for my party. If I want a more damaged focus build, Pyro would be good. If I want a more AOE focused build, Onmyoji is great.
Skill allocation wise, Frost Pillar/Snow Rolling and Ice Wall to me are a must max, the rest is up to you. For Psychokino, Gravity Pole/Raise/Heavy Gravity is a must max. Psychic Pressure lvl 10 should be enough to hit a lot of ice walls in your Icewall+PP combo. Psychic Pressure alone is pretty weak so you wouldn’t use it by itself often. Magnetic Force and Swap is a 1 point wonder, and the rest can be put into teleport for extra mobility.