I believe I know which mission you're talking about and yes, you should be plenty strong to complete that mission. Although, do make sure you have the latest hotfixed version because if this is the mission I think it is, we toned it down a few weeks ago to make it easier.
Another thing you can do is activate the temporary buffs from some of the conquest territories which can give a boost in combat.
If you still can't get past it, can you give me the exact name of the mission so I can take a look at it?