Ice Demon.

I am aware that it has to do with their HP. However I am asking which stat effects it on the spell description. (Which stat should I be looking at if I want my Ice Demon to deal more dps.)

The increase in damage is basically constant as you can see from the chart.

The best levels to cap ice demon would be either where it goes from hitting n number of targets to hitting n+1 number of targets (level 7 to hit 3 targets, level 13 to hit 4 targets etc). OR when the freeze damage lasts longer (level 11 for 10 secs, level 21 for 15 seconds). But I'm sure you know what already

I would aim more to cap at the levels it hits more target because no one really uses ice demon to actually "poison" with its 1k damage per second lol.