[Nagot Gick Fel]

Heh... if you beat his army and stormed his capital as soon as turn 13, he probably needs our help more than you do... :-^

OK, you probably didn't research much at this point. First thing I'd try is to at least spirit curse the bugger (Conj 1, D1). That won't kill him by itself, but it'll improve your chances to cause him a crippling affliction (hope for feeblemind)

Did you research some construction yet? If you have a nature mage (indy shaman or druid or something), have one forge a couple vine whips and berserk pelts and give them to your flying commanders. Improving their attack rating would also help here, if you can. That way they'll tie the manticore down for the whole battle until it autorouts... which it can't do since it's entangled... which usually means automatic death after the turn limit. Very dirty (and quite unfair) trick, but it often worked for me. (Edit: hmmm, now I remember that vine whips require N2 to forge and most indy nature mages are only N1, so probably not very helpful in your case.)