Unless they fixed it for the HD version, there's actually a neat exploit against Vanitas' Lingering Will that works especially well for Terra. Fill your command deck with nothing but Strike /Freeze Raids and position yourself behind one of the rock formations. Basically, you want to have Vanitas running towards you and the rock separating you from him. From there, you can nail him with Strike & Freeze Raids, which not only do a bit of tangible damage, but actually stun him, so you can stay behind the rock and repeatedly nail him with strike raids until he dies. (Raid Commands actually go through the rocks; Curiously enough, this was only allowed starting with the US/PAL release of the PSP version. In the original JP release, Strike Raid stopped as soon as it hit any physical object.)
I think Vanitas' Lingering Will is actually a pretty neat concept for an optional boss in an action RPG. Low health, hits like a truck, but has several glaring weaknesses, like the Stun status effect, and any Aero/Wind based attacks.