Final fantasy 12, how to kill mindflayer?

First I'd say your levels are a little low, I'd recommend at least level 40 for the Mindflayer. As for tactics, save your quickening for the last 15% of its HP, set up priority gambits with chronos tears to tackle stop and/or equip power armlets. Also try and keep your party spread out so his main attack doesn't hit them all. Use physical attacks until it gets down to 20% HP then after it has restored itself use dispel to remove its status boosts, keep pummeling away and then finish with a quickening chain.