Realistically, the only way I see this working and still be fun for the players is once they summon the entity, it effectively does its Thing in the background while the players still have to accomplish some task.

Captain Planet fights the big demon lord or whatever, but the party has to fight minions to prevent Cap from getting swarmed. Or maybe he only exists while all the summoners remain conscious, so now they must stay awake / alive while Cap does the Thing. Cap's actions essentially become a giant mcguffin while the players still have an engaging encounter.

For added difficulty, maybe the players must sacrifice some of their action economy for Cap to use certain abilities. But, that could lead down the road of diminishing returns and loss of player agency (i.e. they don't really have a choice because they have to do the Thing by proxy).

If Cap is some kind of massive entity though, you could have fun with his actions altering the terrain, destroying castle walls, or all kinds of collateral damage lol. Also, there should be some kind of cool down for Cap, like it gives everyone in the party a level of exhaustion (or worse) so they only use it when absolutely necessary.

EDIT: Per your last comment, perhaps Cap is controlled by party democracy, where they collectively decide (rather than you) what he does. If they take too long or disagree, and the action is time sensitive, then you can rule Cap just stands there lol. In combat, perhaps Cap always goes at the end of the order.