Assume there are no other options.
#1 Flareon
- 2449 CP
- LV30.5
- 84%IV 11ATK 14DEF 13STA
- Fire Spin/Overheat
#2 Entei
- 2454 CP
- LV25.5
- 98%IV, 15ATK, 14DEF, 15STA
- Fire Spin/Flamethrower
I don't have the exact numbers. The difference in attack after considering for IV is 7 in advantage of Flareon which is in my opinion not enough to off set the big difference in stamina. On the other hand, Overheat seems better than Flamethrower to me against Blissey.
If you want to specifically power up a fire Pokemon against Blissey, it might be better to wait another month for Blaziken.
Entei's stats are only vastly superior when it is the same level as the Flareon in question. Because their CPs are very similar, that's how similar their stats are, and in that, Flareon's CP is more so based on its attack, so it should be much better. IVs aren't really relevant because of the large distance between their levels.
That being said, Flamethrower's niche is better placed in sweeping teams of weakened defenders that Overheat would otherwise Over-kill. Overheat has tons more dps, and a 1-bar over a 2-bar means you can spend more time dodging Dazzling Gleam, which is itself a 2bar and will be coming out often (especially as Blissey is taking high damage).
