4e actually has good inspiration for this in the form of solos. Salient takeaways: immunities to the most common lose conditions and/or an ability to shake them off quickly, bonus actions to compensate for the action economy being so stacked against them, and ideally out of turn actions to avoid making turns too cyclical.

I mean, an omnigestalt is a trivially easy to make T0. It's also much trickier to balance between it getting dogpiled too early to make a properly climactic encounter, or chewing through PCs fast enough to TPK.