I found Vertrar in the second scenario for FE:LH, but by then most of my heroes were at or near level 15.  Vertrar is only level 16.  wasn't much of a challenge for a party of 8 heroes.

so, I pumped him up a bit...

to level 90.

yup.

at level 90 he has:

1200 hps
97 attack (doesn't change)
165 accuracy (not going to miss)
160 spell mastery (forget about your average magic resistance; you need perfect magic resistance to do much good here)
90 spell resistance (better hope for crits on your spells)

ok, that was a bit more of a challenge.  each of my 8 heroes gained a full level (15->16).

but really?  it was just that... a bit more of a challenge; that's it.  why?  because he is slow (init 18) and because sunder is not resistible, and because you can wear cold protection (which knocks down his attack a lot) and because your guardians can cast immunity while you continue to pummel from afar, summon fire elmentals to harass him, whatever.

even at level 90, I found him less dangerous that the green dragon that had 280 hps. and was only level 16.  dragon is dangerous because...

it is fast.

anywho, vertrar needs some beefing up.  maybe give him a permanent blizzard ability, so that blizzard is cast automatically, EVERY TURN, and he still gets to attack as well.  Increase his init to at least 24, maybe even 28.  Increase his spell resistance to 60, and make him immune to critical hits.  Maybe give him the ability to spawn a half dozen high level ice elementals at random spots on the combat area.  Give him a mass-slow spell.

then, he would be a force to be reckoned with at level 15.