Round 384: Gaylord throws his spitball and coughs up another one. The magma hits! (Rolled 20-4 for long range) It does 28 points of damage, but the ooze ignores the extra fire damage. The ooze has 100 hp left.
The lava continues flowing to 25N10E. They call it Mini Handbook as a joke. The original title was Munchkin Handbook. It seemingly does 22-35hp of damage. That's a lot for CR5. No, that's more than a lot.
I was just preparing to comment on this. CR 3? How many level 3 characters would not get killed by one flaming rock? Also remember that 35 HP is not counting the fire damage, because the opponent is immune to fire.
I just noticed Infiniti saying this thing is CR 5. It is on the CR 3 list in the BoAA, which means it is probably listed as CR 3 in the back of the book.