Things with higher DPS than B/C Gyarados (15.473) against a C/FB Mewtwo:
- Pinsir (+16% DPS)
- PC/SB Alakazam (+12% DPS)
- SA Moltres (+8% DPS)
- Scyther (+8% DPS)
- CB/SB Gardevoir (+5% DPS)
- DB/DC Dragonite (+1% DPS)
You'll notice some of those options are pretty sub-optimal in their own right, either because of having limited use outside this specific scenario or that there are better versions of them available.
Not to tear too hard into B/C Gyarados here: I used one when I had literally nothing better to use. With there being so many better options available it's hard to recommend it unless you're a total newbie.