Once you know about his counter pattern, Dario is a complete joke. Basically, go into battle with red elements all over your grid. Attack him with 2/2/2/FirePillar or MagmaBurst combos until he dies. Seriously, you won't have trouble that way (unless you miss, in which case you can go with the safer 1/1/1/1/1/1/red element combo).
Of course, if you want to be quicker about it (I can hear the pained groans from everyone else right now), start with Magnify-2 and hope someone survives the first turn he gets. Follow that with 2/2/2/FirePillar until you inflict Burns (to increase damage). Afterwards, use 2/2/2/AeroBlaster combos until he dies. The reason for AeroBlaster is because it's a level 4 element (compared to 3 for FirePillar) and has a fast animation.
Also, if you have the Plasma Pistol, use that on him. From what I remember, it seems to always critical on Dario. Use this to your advantage, if you've been using a character with a gun.