To defeat Velius, bring along at least one summoner that knows Lich, a chemist with a Mythril Gun, and anyone with a strong attack (like a knight or a ninja.)

First, move within four spaces of Velius and use wave fist. Then you should get another turn. Use wave fist again and then jump across the water on the right.

I think Velius gets his turn next and he'll use Cyclops. While he's charging, move your guys away from the target. If he targeted a certain person, attack him directly with that person. Hopefully, you'll have a summoner left. Cast Lich (It'll hit him for around 500, and he has 1000hp)

By now he should be almost dead. Just use anything you can to finish him off.