I tried to bribe him just out of curiosity with several of my best diplomats but it didnt work.

I got him with(again), with a newlly formed armey and it was quite easy. I kept him busy with my principes and pedites extraordinarii while i flanked him from both sides with my triarii...only used cavalry for pursuit. Only one man lived to tell the tale he he he
Of course that the fact that I managed fight the battle on mostly open terrain worked on my favour.

This guy( and i could swear there were Ents helping him on those thick forests) cost me me over 4000 men and 3 generals (2 faction leaders).