After my 1:49 5-card run, I did some testing on Seto 3 with my deck, which was this:
- 13 dragons (4 big; doesn't include Thunder Dragon)
- 7 thunders (1 Thunder Dragon)
- 4 equips (DT x2, BF, HoL)
- 3 Umi
- 1 Mountain
- 1 Raigeki
- 1 MBD
- 10 fusion material (1 Time Wizard, 3 rocks, 6 plants)

I dueled Seto 3 a total of 76 times and won 24 times, for a win rate of 31.6%. Perfect execution would maybe have gotten me an extra win or two.

Then I changed my deck, replacing Horn of Light and a Dragon Treasure with 2 Megamorphs. I dueled Seto 3 a total of 25 times and won 21 times, for a win rate of 84.0%.

I think the FM developers did a good job play testing for difficulty and balance... except with Megamorph. It is even better than it seems.