The elemental adds are what use sleep. Thus you need to kill them asap.

I recently did this fight, but I was a much higher level than you (~30), so it was pretty easy.

What I did was gather up all the elementals and thundera them twice. They died in 2 hits (overkilled on the 2nd, so shouldn't be too hard to do it at a lower level w/ same equipment). Yes, my caster did get hit by the reflect from mateus. I just healed through it. You have Decoy, so that person shouldn't be takin any other damage.

After adds were down, I put reflect on my nuker, set a gambit to cast thunder on herself, and left her alone. My tank was doing fine w/ main gouche and ice shield, and my healer even had time to get some hits in with her gun. Once the adds are down it's a very simple fight. Even better if you don't have 2 casters cause the sap from the critical/overdrive/whatever thing made my healer start healing the reflected person (not good, had to turn off those gambits or manually stop her until I potted the nuker up), thus if you don't have to use reflect on yourself it will make it a lot easier.