As stated by others using the QWER keys is key, be patient, if it looks like the mystery wasn't selected when you press don't worry it still goes through, the instance is just a lil buggy. Do not press a mystery key repeatedly during a round or the game goes derp and makes your attack miss. Once you're past the summon troll, just bash your head against the pains till you get through, it is not worth forfeiting after getting past the summons troll portion...

Subnote, it is wise to over grind so you only have to deal with the glitchy rage inducing summon portion once and move on with your life.