Yes. He's dominating Darth-level opponents with ease.
http://www.comicvine.com/profile/darthant66/blog/darth-marr-respect-thread/103084/
Originally posted by DarthAnt66"With his humanity long forgotten and his face unseen, Marr's inner thoughts are impossible to determine. But his desire to leave a strong Empire behind him is unquestioned, and his abilities are second to none."
http://www.comicvine.com/profile/darthant66/blog/darth-marr-respect-thread/103084/
―Darth Marr Codex Entry
One would imagine that places Marr above Malgus.
And we haven't seen much from Lachris, but that's no basis to write off her abilities, she is a Darth for one, and apprenticed to one of the most powerful Sith in the Empire. On the other hand Malgus has been challenged by several high profile Knights and Masters.
That's a reference to overall ability, not necessarily Force ability.
Lachris has no feats or anything to suggest her power is on par with or even approaching that of Malgus's enemies like Zallow or Leneer. Could she potentially be comparable? Perhaps, it's just there's not much suggesting that. Darths aren't uncommon in the Empire, and your master doesn't dictate how powerful you are at all, or else all of Luke Skywalker's students would walk over Obi-Wan Kenobi. Likewise, Yuon Par was a pitiful Jedi in terms of combative ability, yet the Barsen'thor was among the most powerful Jedi in the Order.
For that matter, Exal Kressh was apprenticed to Vitiate himself, but I doubt you'd make the case that she could match or beat Malgus in a Force contest.
That's hardly enough to confirm his victory in this battle, though. It definitely doesn't have more worth than the quote stating Malgus's battlefield feats have never been replicated.
The gist of your point regarding Marr's dominance over Lachris is noted, but I've seen nothing to suggest Malgus wouldn't do the same. Is there anything to suggest Malgus wouldn't fare just as well against Lachris?
Fair enough.
Well again Malgus has been challenged by Zallow - a Jedi Master - and Leneer, a Jedi Knight, both gifted yes but surely Lachris, a Darth apprenticed again to among the greatest Sith in the Order, is a match for them.
EDIT: Lachris also has a battlefield reputation of being undefeated.
If I recall Lachris is taking down by the Barsenthor as a Knight(?) who is pretty powerful at that point. A presume she was a challenge.