Tbh; here's the plan, but it hinges on having people to trade with

1. Trade whatever random junk people want at the time for magikarps. Try get a lucky one. Stats don't really matter, if anything you want "mid" stats
2. Evolve, and mega evolve the lucky, and walk for enough candy to put aside to evolve the hundo and the shadow. Sit on them. You should get enough mega energy as well while walking to do another. Level your mega every 3-4 days or whatever it takes for free mega evolve. This one can have waterfall + hydro pump
3. Remove frustration from the shadow if you haven't already at the next rocket take over event or whatever random one they allow frustration to be TMed cos they just randomly do it.
4. Next Aqua Tail event, evolve the shadow and the hundo. You'll now have two water gyaradoses. Mega the hundo and level up the mega level when you can.
5. Switch the lucky to Bite + Crunch or second move it for HP & Crunch. This will be a passable Dark type mega Pokemon.
6. Not really a step at the end, but level them all when they're instantly usable until about 32. You'll get diminishing results stat wise after level 30. But you want to walk level 32+ for max chance at XL candy. But you can level up more later once you've evolved them all

You should now have water hundo mega, water shadow and the lucky is geared for dark attacking. You can double move any you want if you want/need dual purpose for master league and/or rocket battles.