Maybe about 3-4x the hp of Cinderwing (Boss).
His only attack: Gae Bolg, a 40k AOE damage spell, High damage, High accuracy, though you still have a chance to dodge it if you manage to get an AGi- debuff and have a high AGI figher in your battle party.
His armor and magic armor is through the roof, Using a Hammer wielder against Xalvador is nice due to the crushing blow (skill) being able to place a Pdef- on him, an albeit slight debuff that can help your physical damage dealers wittle down his HP just a tad bit more ever so slightly.
Elemental magic does next to nothing, but their status effects still apply to him just like any other mob. (Discovery?) Upon Monday, September 4, 2017. Xalvador is immune to Paralyze, Burn still works, Frozen has yet to pop for me. (WIll update results)
You get 3 rounds of free hits before he starts to attack you. In which case all he does is Gae Bolg, If you manage to dodge his first Gae Bolg, Xalvador will continue to use Gae Bolg until your party is wiped.
(stun locking, as cheap as it might be, may be the only way to kill him without you getting rekt.) As of September 4, 2017, Xalvador is immune to Stuns and Confuses as well.