On the first boss id recommend being levels 5-7 first when you enter the battle there will be two other minions take them out with a hero action.  Next use another hero action toward the monster and do scratch damage then take your HG person and and use a her action to try and line up for a tri-attack you should have 2 or 3 bezals then 1. get rid of the oil thing so he cant refill Hp or go in and try lining up tri-attacks and you should be done in 5-10 minutes

On the second boss use hero actions to get behind the barriers and cause scratch and direct damage to chip away at the shell of tortouse when the barrires are down go and try using hero actions in desperat need andd just attack the core of him if e keeps blocking hos core with left over shell back away the walk near and he will leave a huge chunk of damage available. Also be levels 10-12.

On the third boss be levels 13-16 and set up attacks and get behind barriers and do not take out his minions unless you need more bezal then once his defense is down attack him with all the power you got and do not come close to him with hero actions he will swing you out of the air even if you swear he didnt hit you.

On the fourth boss keep your weakest HG in front of the statue so it wont move then take out the other enemies with scratch then direct you may have to spend 50k to heal the statue once but dont worry you can go to a nearby store and sell all the item of scrap you have and when you get to the boss you should kill it in two-three rounds. Be levels 20-30