Hi, and welcome to competitive battling! Your team isn't that bad, but there are definitely many improvements to be made. Let me give you a quick rundown on what you could fix up.
First, Ampharos = fast??? Ampharos's Speed actually gets worse once it Mega Evolves and can be outsped by practically anything. Give it a Modest nature and invest its EVs in HP. Good job with the moveset, though.
Darmanitan is a pretty bad Pokémon competitively, as it's outclassed by many others with similar stats. You definitely should replace it with something else. There are so many UU Pokemon that could fill this slot better. If you would like a suggestion, leave a comment asking for one and I'd be glad to help.
Great Nidoking set, except switch Flamethrower for Ice Beam, so you get BoltBeam and just better coverage in general, and give it a Life Orb and a Timid Nature.
Cobalion's set right now is pretty mediocre. I would either go for a full offensive set or a full staller set. If you want to go offensive, do Swords Dance, Sacred Sword, Stone Edge/X-Scissor, and Zen Headbutt. For a staller set, do Stealth Rock, Thunder Wave, Close Combat/Taunt, and Volt Switch.
Great Trevenant set. Give it an Impish Nature.
Great Milotic set. Give it a Calm Nature.
So that's my opinion on what you could do to fix up your team. If you have any questions or suggestions, feel free to comment and I'll do my best to get back to you. Good luck battling!
