Do you like to use an Anti-Party?

[(Psi)SeveredHead]

I had fun with this in 4e.

I did a Dark Sun campaign some years back. The penultimate encounter involved an NPC party, where each PC-equivalent was an elite. It was a nail-biter. Turns out NPC elite leaders (yes, I used a little healing) draw so much hate, but they cannot be killed in one round by an optimized striker, which always seemed to happen when they weren't elites. (Also making Healing Strike a recharge power was too much of a good thing, even though the healing was much less for the NPCs than for PCs.)

I liked doing this in 3e as well.

 

[Eltab]

Tomb of Annihilation is well-designed for an anti-party, where the PCs get hints and clues that somebody else is wandering the jungles too.
But I would not have the anti-party targeting the PCs specifically - rather, like the 'Yellow Banner' group, they are trying to also find the Hidden Treasure.

P.S. In one ToA encounter, the enemies specifically behave like how Belloc treats Indiana Jones in Raiders of the Lost Ark.