If you want to build a hyper offense, one of the Pokemon that i'd definitely look at first as something to build a team around is mega lucario. Mega Lucario comes packed with amazing mixed attacking stats, a great/trolling Speed stat once it gets mega evolved, and a great ability in adaptability that provides it with 240 BP Close Combats and 160 BP Aura Spheres and Flash Cannons to break down bulky threats with (along with an 80 BP Bullet Punch and Vacuum Wave, the same BP as extremespeed). It can run both a physical and special set effectively, which adds to its unpredictability and makes it even harder to handle, especially considering special megaluke threatens manly of physical luke's checks. All in all, Mega Lucario is a frail but incredibly powerful offensive threat that serves offensive teams pretty well. As for teammates for mega Lucario, it really depends on what set you're running, and in physical's case, what coverage move you're running. If you're running physical mega lucario with Crunch, Pokemon that can easily deal with Pokemon such as Gliscor and Landorus-T are very helpful for Mega Lucario, as they can take any move from SD Crunch Lucario and KO Back with a STAB EQ. Rotom-W, Gengar, Keldeo, Greninja, and Mamoswine are good examples of Pokemon that can break through these Pokemon, some can switch in safely and some can't, but once they get in they should all get the job done. If you're running Ice Punch instead of Crunch, you'll definitely want something for Aegislash (either way you want something to wear it down tbh, since it can take +2 dark moves from both physical and special). Reasonably bulky things with STAB EQ in general are pretty good against Aegislash, as well as Bisharp (personal favorite of mine) who can threaten aegislash as long as it doesn't get in on Sacred Sword while ignoring King's Shield drops with Defiant (bisharp discourages defog too, to help you keep rocks up). As for special Mega Lucario, because it completely ignores things like Gliscor and Landorus-t's physical bulk and can cleanly OHKO both with +2 flash cannon, Pokemon that benefit from having those two out of the way are natural teammates, such as Garchomp.
hope i helped! :)