Thanos, Ion, a Guardian, and Adam Warlock with extensive prep.
Thanos and Adam Warlock both have pretty much more experience with the IG than anyone else, and Thanos is the only one to have the HOTU. Together, they know what SMP is going through more than anyone else having given this much power. Together, they might have an idea on how to trick SMP into defeating himself.
Ion and a Guardian would know more about SMP, and would know enough about the rings to find a trick around them.
SMP is mental mess. He's not going to be able to handle getting all this power. If anyone, these guys could trick him into defeating himself.