Well, you still need to confuse/charm Skeleton King (or the minions) so they attack each other. He won't attack them just like that.
Oh yes, he will Use max toughness gear, Molten gem and that sort of stuff, so he can't kill you, and just stand in there... he'll continuously cast his attacks, and the cleave-like attack will damage all his adds. It's just a matter of time (and survival, and no thorns damage :P)
You don't need charm or anything like that.
Pretty much if you can tank his swats you just move next to his adds and he will eventually do it with you doing much. This is either the same one or a variation of a standard achievement.
You know something about feats of strength in season 4? I only got "reach lvl 70 in season". but the bar is only 1/3 full so I think 2 left so far...
Feats of strength are generally either special event achievements or archived ones that no longer count toward the active point total. There is no specific quantity for these. The number will vary depending on each players circumstances. The bar means nothing. It is only showing that you made progress not the amount of progress. At the end of the season your journey achievement will be moved to this area.