Okay, your Spore strategy is much better than your Endeavor strategy.
Unfortunately, this strategy will probably not work.
Why? Because Smeargle has no "powerful attacking move[s]". Although the painter has access to 99% of all Pokémon moves, he can't use any of them with his abyssmal attacking stats. Even a Dragon Ascent won't hurt anything coming off of a base 20 Attack stat. Smeargle's best use is really just to Shell Mash + Baton Pass + Spore for Singles and Fake Out + Dark Void for Doubles.
So to answer your question, the optimal EV spread is probably 252 HP / 252 Spe / 4 Def.
Sorry to crush your hopes and dreams :(