I use a Monotype Steel Team which has done well in some competitive battles, monthly competitions, and the Battle Tree, utilizing my favorite Pokemon. Probopass, Klefki, Bronzong, Ferrothorn, Forretress and Mega-Steelix; I also sub in Magnezone for sheer power and Aegislash for some great physical wearing-down. I've also gotten over my pride, somewhat, on not using Legendaries, but mostly to give Celesteela a go.
As a result, Fire, Fighting and Ground attacks are the bane of my existence, lol, but I have plenty of tactics on dealing with those shortcomings, since I've compiled my weaknesses and adjusted my team to deal with them accordingly.
Personally, my weakness is likely being stubborn about not using pokemon because they're super high ranked on Smogon uber tiers or whatever; I fine-trim my EVs and IVs, natures and such to optimize their combat potential, but I usually stick to the things I like in order to be successful. Gaming should be about having fun, right? Also, I usually think Legendaries are cheap and too frequent online for easy wins, so I usually don't use them.
I think Celesteela is cool though, in terms of design, appearance, and playstyle. That's as far as I may go though!