Once we went through the portal, the combat was very challenging and unavoidable, regardless of the party's composition. The difference is that the high level mega battles were in furtherance of the plot, which was ultimately concerned with with recovering TNO's identity and in the process trying to answer the question that served as a central theme of the game.

Extraneous mega battles being added for the pure sake of awesomeness goes against everything that Planescape represents as a setting; that kind of mentality would turn Planescape into a monster hunting session wherein the PCs get to take a crack at the powers. Hell, the setting material for Planescape (which I have been reading in preparation for a campaign I am starting wherein the PCs are all Primes who inadvertently wander into Sigil and have their perspectives and values challenged) flat out states that Planescape games are meant to revolve around the ethics of the PCs, first by letting them act in furtherance of their beliefs, and then once they grow more powerful and are able to set their own goals start challenging those beliefs, and warns against using it as yet another standard setting that serves to provide magic items and gratuitous ego-stroking mega-monster hunts.

If more battles were to be added,they would need to rise above the standard "kill unkillable badass X to prove how badass you are" that plagues so many RPGs, and instead be done either in furtherance of some larger goal relating to philosophy in the Planes (taking down or aiding a high level revolutionary holed up with an army bent on attempting to break the hold of the factions) or deconstructing the values of Prime adventurers (giving the player a giant dragon fight which results in the Harmonium hunting the PCs down for murder). If all people want from the Planes is more awesome monsters to fight so that they can feel like Billy Badass, then they would be better off sticking with Forgotten Realms, which has all the philosophical preoccupation of a dishtowel and prioritizes finding any means for giving the PCs new awesome magic items over lore (the moment I saw that the late Dak'kon's sword was purchasable despite it being established that the blade would disappear when he died, I had to resist the urge to headdesk). Part of the package deal with Planescape is playing in a setting with differing priorities; asking that those priorities be ignored is a bit like someone knowingly signing on for an RP intensive game and then getting bored and asking when they get to kill things, or walking away from Spec Ops: The Line and saying that what would have improved it was the ability to get a happy ending.