I am assuming this is a 1v1 fight?
Morgoth.
Would this ever have
remained a 1v1 fight? Think of what happened to
Númenor at the end of the
Second Age ...
Eru occasionally (once certainly) intervened ... though I would give him (Eru) very low marks on subtlety before
the Third Age ... and even lower marks for dereliction of duty ...
Then, there's
Tulkas. Late arrival to
Arda. Utterly single-skill
Vala. But what was that skill? Slam-dunk (basketball). body-slam (professional "wrestling") etc. - or basically beat the 💩 out of
Melkor / Morgoth at all times (except perhaps in JRRT's abominable late writings).
Ungoliant had to spew out clouds in
Valinor after she had destroyed the Two
Trees (which - apparently even JRRT failed to notice this - should have
massively weakened her) - mainly for what reason? Tulkas would have torn her to shreds had he caught her (and Melkor wouldn't have looked very healthy after Tulkas got through with him ...).
Then there is the point about the effect that entering into Arda and taking bodily form has on the
Ainur. Carl F. Hostetter's NoMe adds some interesting facets to this topic (mainly, IIRC, about
Melian). But considering the title of Christopher's 10th volume of HoMe, and the statement "all of Arda was Morgoth's Ring", it would depend on
when a confrontation between Morgoth and
Manwë has taken place. And if Morgoth could have been able to affect Manwë at all - I'd think the latter would have replied to any such attack with "calm down, bro",
without being affected ...