there's nothing they can do with prep to defeat spawn
Drain him, BFR him into a place he can't escape from, amp themselves to where they're more powerful than him, neutralize his powers, take control of him and remove him from reality outright. It seems like you're overestimating Spawn and SEVERELY underestimating what all these two have accomplished with time to prepare.
the only one that would be any challenge whatsoever would be doom
Funny because Reed has beaten Doom on several occasions with and without prep.
and even then spawn and his suit feed off of evil and sin, therefore doom would be significantly empowering spawn and spawns abilities the entire fight
Only certain versions of Spawn function that way, and not in such a manner that'd be relevant against Doom.