For Mars I would suggest it is located outside the land tank, when the boss spawns the door closes before players get there to see it and when the boss actually makes it appearance the entry to the land tank hisses with pneumatics and the boss makes its way down the ramp escorted by an elite phalanx squad, this squad would be projecting a bubble shield onto the boss and must be taken down before the boss can be harmed, also to harm the boss you would need to shoot down one of the two elite centurions (these centurions would have unique jet packs that allowed real flight instead of just jumps) and grab an EMP or something that the player who carried it would have to sneak around behind the boss while their team distracted if and plant it on the boss locking up the bosses mech armour allowing players to fire on it until the cabal war mech started up again. After doing this a new wave of elite cabal would come the the mechs aid, and after enough damage a new mechanic would be added. It could be called a cabal Zeus mech or something, I don't know..... Other boss area could be the black garden entrance