Sersi gave a near matched fight with Exodus and he didn't defeat her per se. He knocked her into the rubble and left the fight. She was up in the next panel ready to continue.
Legion's encounter with Magneto wasn't really a fight for both sides, so that's hard to classify. He did overwhelm a group of X-Men and Bishop took down a disoriented Legion who was not fully attuned from the restoration of the world after the AOA incident.
The reason this match-up is unfair is because you have God-like cable who has full control and full power, and you have nate grey prior to his shaman days. nate grey didn't even reach his full potential in his shaman days. this is a very biased match-up. to put it in perspective in you cable heads...its like if i said full potential nate grey vs regular cable.
if things were FAIR, then all the mutants would be at the same level of handle on their powers. that is why you should say they are all at full potential so there is nothing left unfair.
Yes it did take both cable and nate grey to beat exodus, but lets not forget nate grey was young and could only control a portion of his powers. and obviously cable had the techno-organic virus.
In my knowledged and fair opinion, i would first make them all at full potential. then, put cable and nate grey at a tie. why you ask? because
1]. Cable had askani training that probably gives him a little more control of his powers.
2] Nate Grey was never shown at full potential but was supposed to be more powerfull than a virus-free cable. So that rests at a tie...since Nate is long dead or dispersed it is more reasonable to say Cable wins...but technically it would be a tie.
Cable/Nate, Legion and then Exodus. Legion has always been classified to be more powerfull comparatively than Exodus. lets not just vote for the under dog just to do so.