A year ago but I'll still reply. Imo main thing rn that's helping me push through is OP tanks and just rushing to the objectives during hard levels.
OP tanks are extremely key. This game allows you to get some 20 hp tanks that are absolutely brutal. Especially with the "overrun" ability, they are able to take out 5 troops by themselves. During the Novogrod scenario, I got absolutely destroyed the first time through because of the sheer number of troops. I reset myself to Barbossa (or the one before Novogrod, I forgot) and developed one 1000 prestige tank. Starting from the south side, I was able to clear out the majority of the troops with a single tank, as it could overrun everything. Had like 7 turns left over till end of the brilliant victory timestamp.
Second thing is of course, objectives. There are some scenarios that you really just can't beat if you go for all of the mini objectives. Take a few fast moving troops and just rush to the objectives. You can kill off all your auxiliary units, you can be surrounded by enemy troops on all sides at the end, but a victory is a victory lol.