Best way is to go brute, if you ask me. StJ does not rely on self damage buffing so doesn't take much of a damage hit by going brute (unless one is planning to go all out with Musculature, Assault and global bonuses), you may even see a significant increase from things not running away, getting more damage out of Burn and Blazing Aura, or even melee attacks generally (even a mere 100ms spent repositioning is a significant damage loss if your power animates in 1s).
If you're set on a scrapper, well, other people will have to help you. Never been able to make a /FA scrapper that worked myself, due to the aforementioned running issue coupled with the awesomeness that is Burn, leaving me to feel like I'm missing out by too much compared to what I could do on the brute version. The most prefered forum strategy seems to be softcapping to S/L, with perhaps some E/N defense. I prefer to go with 20% positionals and rely on lucks.
I have made Fire Armor toons in the past and I just can't seem to figure out how to IO them. I can never hit soft caps in defense to melee or smashing/lethal without making sacrifices to other areas that make the toon less fun. So I try light defenses with extreme recharge and I'm not real sure I like that. What kind of #'s do you guys like to hit, obviously you can't make the set take damage like other beefier sets, it's about damage, but still I feel like I don't know what I should be aiming for.