Keep in mind Crit applies to DoTs as well and you will landing a lot of hits/dmg on DoTs so the more you get to crit the better until cap as Kevee mentioned.

There is a trick to call your summon with acc gear then switch to INT>DET>Crit set.

For me Acc it s pretty useless unless you go to Coil where you need Acc for sure.

Shadowflare it s a must as long the mob stands to it keep in mind it s 250 dmg based (and has the same casting time as ruin) so Shadowflare UP>Ruin.

Thunder Rotation to each mob after your General DoT rotation helps a lot on AoE DoT DPS but never forget Shadow Flare

I Agree with Ruin 2 part altho keep in mind that you can use 2 skills at the same time by using ruin 2 (through skills that ignore general cooldown= Virus-Fester-Energy Drain and others) than just ruin 1 if you have mp to spare of course