Attack enemies in the same order. Usually, that is left to right. Sometimes, it is dependent on elemental weaknesses.
DO NOT USE YOUR CHANNEL BEFORE THE BOSS STAGE! You only get one channel per life; unless you die, you get only one channel for the whole quest. As reviving requires you to use a quest stone(which costs a dollar a pop) dying is not recommended, even if it would afford you multiple channels.
For all stages other than the boss stage, use normal attacks even after skills become available. Save your skills for the boss stage. The only exception is healing skills; if it is necessary, it is ok to use them.
During the boss stage, immediately channel the strongest player. If you are the strongest player, use your best multi-hit skill immediately. On the second turn, the strongest player should immediately channel the second strongest player, who also uses his/her best multi-hit skill. After that, it's business as usual, although here you should use all of your remaining skills that you have been saving for this stage. Use those skills before using normal attacks during this stage.