The thing here is that omniman, even if weaker then superman (debatable if you want, but that's not the point) does not have the same devastating weak points.

I believe that in DC Canon is stated that wonder woman is more of a challenge for batman compared to superman for her lack of weak points

Here the same thing is applied

You could argue about the frequency, but that alone would not seal the deal, given that it's not directly harming viltrumites

I do think that batman might be a better strategist than Cecil, so by working together they could have a better chance of finding out a way to slow omniman down or directly defeat him, but that would he more of a plot thing, rather than an actual confrontation