Ultimate Magus between Wizard and Warmage. Residual Magic feat at 9th.
This build only gets obnoxious at 9th level, TBH, and even then only kinda.
Feats: Residual Magic, Empower Spell, Quicken Spell, some method enabling practical of Quicken Spell with a spontaneous casting class. Energy Substitution and Searing Spell are both useful as well.
CL 11 is obtainable with an Ioun Stone. Don't pass it up, it boosts your Scorching Rays GREATLY.
Round 1, standard action: cast Seeking Ray. Make it a Energy Subbed [Fire] Seeking Ray if you like. [4d6]
Round 1, swift action: activate Ultimate Magus ability to burn a 4th-level spell to cast a Quickened Scorching Ray at the same target. [12d6]
Round 2, standard action: activate Ultimate Magus ability to burn a 2nd-level spell to cast an Empowered Scorching Ray [18d6]
Round 2, swift action: activate Residual Magic to cast a Quickened Scorching Ray [12d6]
Round 3, standard action: activate Residual Magic to cast an Empowered Scorching Ray [18d6]
Round 3, swift action: activate Residual Magic to cast a Quickened Scorching Ray [12d6]
3 rounds, 76d6 fire damage, Seeking Ray practically guarantees that none of these attacks miss.
You will also likely apply your Warmage Edge 6 times over these 3 rounds, so total damage can near 300. When you get more 4th-level spells, you'll probably want to use Twin Spell instead of Empower Spell.
At some point, go take the War Mage mentioned above and your damage will quickly out-pace the BSFs against anything not immune to fire.