Which Magus are you talking of?
Anyway The In Betweener is gonna taking this.
He can obliterate Ego pretty ease, this guy can manipulate time and space with a wave of his hand and his control over cosmic power/energy is higher than that of Ego, he's also above the Watchers, maybe a dozen or more of them can take him out aided from Ego and Dormammu, maybe.
Dormammu himself was beaten from Strange in the past, the IB outmatch him easy.
Dormammu has only been beaten with cunning but is extremely powerful.He comes from a dimension outside Eternity,and I doubt the IB would have power over him.Ego is an elder of the Universe.
And the IB sucks.
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In Betweener is both strength and weakness so he could beat one of them but he would then have to lose afterwards. Ego is below Galactus's level but only becuase Galactus doesn't use his full power very often. However all of the Watchers together would triumph
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