Well to be exact, he is a raid boss, if yku don't have the raiding party achievement and you go and kill him with 2 clan mates, it give you the achievement, I actually did this yesterday to test it
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To be exact Skolas is NOT a raid boss and has never been a raid calibre event. The 'raiding party' achievement is misnamed as any fireteam consisting of all clan members can get it from doing an event together. The Flawless Raider achievement requires a flawless raid and can never be achieved in PoE.