As a general rule of thumb try to have your combo under 100 after using a skill, then bring it back over 100 for maximum combo attack/combo critical. Combo drain at 120~129 combo with 129 being perfect, then you're at 99 & 1 hit away from getting back to max combo critical/combo attack. If you accidentally go to 130, use combo drain twice. 190~199 use Combo Fenrir, If you need to buff both use Combo Fenrir first so that using drain doesn't weaken you're Fenrir, so get to 220~229 combo then do drain->Fenrir (assuming no points in Tempest (if you have points in Tempest you'll need to Fenrir at 190~199, then get combo back up to 120~129 and drain). Combo Barrier is best used at 290~299, and finally if you want to Barrier & Drain get to 320~329, then use those 2 skills in any order.