You may want to consider taking a look at this page.
Since you're a Guardian, you can do the Secret Mission to get momentum. This will help you greatly.
If you want to defeat it without momentum, you can try using the full Horo-Show Void set to take -25% Void damage on his Void attack. Go into the battle with a full SP bar (use Essence Orb to make sure to do this) and spam the skill. Use Underwyrmling to lower the number of SP attacks from it.
Also, Voidwraith is undead, so you can use the Artix ally assist to deal good damage. It doesn't resist Void damage apparently.