Wait wait wait.
How on earth is Mephisto smarter than Thanos?
The only way Thanos would lose to Doom and the FF is via unwittingly WILLING himself to lose. He does this quite often.
No way Doom beats Thanos head to head, with or w/o prep time. His "defeating" the Beyonder is overrated considering the Beyonder was still relatively ignorant. He only stole Galactus' converted homeworld because Galactus did not consider anyone on the planet as worthy of his notice (hence him sending his "watchdog" down in the Secret Wars mini-series). Hell, Thanos with only the Power Gem defeated Hulk, Sub-Mariner, Wolverine, Cyclops, Thor, Iron Man, Spider Man, and Doctor Doom. Shortly thereafter he defeated Galactus, Lord Chaos and Master Order, Two Celestials, Kronos, Eon, AND Eternity.
Recall him fighting Tyrant to a standstill? He did not have the IG; he had a bauble imbued with Morg's stolen power. Tyrant...is um, NASTY.
Doom and Reed are not Thanos' peers in prepping for a battle. Doom/Reed are more on par with Batman (YES, he whom devised the means to defeat Mageddon, who is far more powerful than anything Doom faced) and Black Panther (whom devised the means to take down Galactus).
Thanos' peers would include Darkseid.