And what else did he pump up besides his direct skill? Just curious...

The most difficult thing was to max out Stealth (I had to sacrifice a few chests to pump it up), and then it was just a matter of technique. Then I fully pumped up combat:D In my opinion, I pumped up magic of Chaos to level 3 and a little bit of magic of Life.
The point is that on the map there were many level 4 creatures guarding cool artifacts. At the beginning, they gave up to 2 levels (if my memory serves me right:D)
So, I ended up with a pumped-up fighter with Logistics and boots, and cool artifacts:)
And the MAIN heroes didn't even manage to fully pump up their own branch:D

Did you come to him without an army? on XL? well, then you are just... it's scary to play with you:D or the opponent messed up:)

We were playing on Champion difficulty, there was no money for an army or buildings, and the Rogue was just running around and pumping himself up:D