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I'm a high level competative player and and i haven't used him. but looking at his stats and moves now... I'd either run him with full HP and Atk investment, shadow claw and horn leech for STAB and maybe will-o-wisp if you are good at predicting switch ins.
A more gimmicky but possibly much better set could be to run him as a subseeder with substitute, leech seed, will-o-wisp/toxic and horn leech with the harvest ability and sitrus berry. It's a pretty common end effective Exceggutor set in NU. If you have a sun team this would work even better. Put growth on him and watch those stats grow. Pity he would still be oneshotted by any decent super effecive hit.
Use what you like though guys. I always do and while it's a bit harder sometimes you can shit on any Aegislash faggot if you hone your instincts.