I say CR 3
"Guardian" Fiendish Pseudonatural Fire Elemental: CR 2; AL CE; Size S; HD 2d8; hp 0; Init 5; Spd 50 ft.; AC 15 (Touch 12, Flat-footed 14), Slam +3 (Slam 1d4/Fire 1d4); SV Fort +0, Ref +4, Will +0; Str 10, Dex 13, Con 10, Int 4, Wis 11, Cha 11
Languages Spoken: Ignan
Skills (ranks are in parenthesis): Spot 5, Listen 5; Improved Initittive, Weapon Finesse (Slam)
Special Attacks: Burn (Reflex DC 11), Smite Good (1/day +2), True Strike (1/day +20 to attack Insight Bonus)
Special Qualities: Fire Subtype, Elemental (immune to critical hits, poison, sleep, paralysis and stunning), Darkvision 60', Cold/Electricity/Acid Resistance 5, SR 8, Alternate Form (-1 moral penalty to attack rolls/Standard Action)