Start with the Scrapper/Tanker/Brute Fire Melee set. Move the slider to lvl 50 xp. Pick the powers that only do fire damage, like scorch, fire blast, incinerate (or combustion? one of those has some smash dmg IIRC), breath of fire, etc. Once you have 100% XP at 50, move to the secondary set. For bosses, you might not cap XP with just the primary.
In the secondary, I like to use super reflexes and take Enhanced senses to give mobs extra PER so they herd easily. You should be able to get to 100% XP for minions, lts, and bosses with only fire damage and enhanced senses. I don't bother with EBs and higher.
Also set the mobs for melee, no flight, etc.
Then start your faction with 3 minions, 2 LTs and 1 boss. That should give you the best spawn size. For more variety of costumes, the ratios seem to affect spawn size, but I haven't worked out the ideal ratio overall. So I try to keep that 3:2:1 overall (e.g., 6:4:2, 9:6:3), but there's some trial and error there. When in doubt, I add minions and LTs before bosses.
If you don't want to boost perception, just pick a secondary and give them the weakest defense to some non-fire dmg. The move boost in Electric Armor, another fire only attack from the blaster secondary, etc. You could give them a trash S/L defense or resist, but that can hinder any non-fire farmer hitters that might join you. If you need to get that last bit of XP on a boss or EB, even build up is okay, because buffed mob ACC and DMG won't matter if you're capped to their DMG type.
HTH