Unless Doom does something stupid enough for Diablo to make him beat himself, I just don't see Diablo outsmarting or outgunning Doom in anyways. He's a crafty opponent, but not Doom level, at all.
He has some awesome feats like getting around the enchantment of the Destroyer armor and hacking it with alchemy and reversing a magic spell that summoned Shumma Gorath, but those are far in between. The last time he attacked Doom he used... Fire.
His other better showings are during Busiek's Avengers and in one of the FF issues just before Civil War I think. He's among my favorite FF villains, such an interesting powerset.