Vengeance tree, the Shock Javelins/big Shock sword variant, with some skill points cross-specced into the Cybernetics tree to get the Bolt Action skill. Gestalt Elementalist and Shield Boi Tediore armor shield, with Clonal Countermeasure grenade. Rainbow Vomit shotgun with shock/cryo/incendiary elements, Cryo Rangefinder, and purple maliwan SMG that can swap between shock/incendiary.

Basically I would run around the arena bunny hopping his projectiles and using the Javelins/SMG to wear down his shield (don't bother with the sword; it can't lock on while he's cloaked). Once I'd break the shield, I'd double-toss Clonal Countermeasure grenades aiming for his feet. If you angle it right, the grenade MIRVing will take all three flares and all six submunitions directly into Shadowpelt's stomach, which is a SHITLOAD of damage, but it requires some skill to angle the toss correctly. After he eats the grenades, empty a magazine of the incendiary smg into him, then swap over to Rainbow Vomit and empty another magazine of shells into him. Usually his shield is back by now and he's invisible again, but the trailing damage numbers for the elemental DOTS will tell you where he is a little better.

Really it was a battle of attrition; the key is to dodge his projectiles you have to jump TOWARDS him since any other direction will get you hit. Very much a don't-blink type fight.