Really?The only time i've seen mephisto go down in his won realm is when A):The beyondersbanes exploded(SWII) and B):To adam warlock and the soul gem.And even then it was just a test of who had a stronger control over soul.
__________________ Some men, just want to watch the world burn.
The question is how exactly Dormammu and Mephistos combined realm would affect the powerlevel of especially Mephisto, Dormammu seems (atleast to me) as being better at keeping a high powerlevel when leaving his dimension in comparison to Mephisto, while Mephisto can hold a very high powerlevel while within his own dimension (if written to it).
However giving Thanos and Odin 10 days of prep against two beings they more or less knows the full capacity of is imo a bit overextended, one would have given Odin and Thanos a real challenge, 10 just swings it to much in their favor from my perspective.