Swampert
I wouldn’t choose M-Swampert for this team, since its only really viable as a rain sweeper. Your team doesn’t synergize with rain (Arcanine) so there’s honestly no point in using it. Plus, you already have Mega Salamence.
Greninja
The problem with Greninja is I literally can’t tell if Salamence and Arcanine are running physical or special sets. If you provided that info I could 1. suggest a different Pokemon for this slot 2. keep what I’m about to say.
If this is Sun or Moon, great. You can get the demo to get a Battle Bond Greninja. If you are playing USUM, you should be looking for a Protean Greninja. Both abilities are quite good (and much better than Torrent).
As for nature, try to get Timid (-Attack, +Speed).
EVs should be 252 Special Attack, 252 Speed, and you can throw the extra 4 into defense or special defense, it doesn’t matter much.
For moves, try Surf/Hydro Pump for STAB (Surf
for accuracy, Hydro Pump for power), Dark Pulse for STAB (by the way, I’m basing STAB off of Battle Bond, if it has Protean everything will be STAB), Ice Beam/Extrasensory for coverage, and then the last slot could be Water Shuriken (priority)/U-Turn (pivoting)/Grass Knot (coverage).
Oh, and for and item, try a Choice Scarf/Specs for extra power/speed (works well with U-Turn as well).
~~~
So, in conclusion, I’d go with Greninja.
Hope this helps! :)
