Short Version: Casual, Allfather's Garden, Level 10, must kill the Bloodmount with flamethrower not the rider to count in or to count, die or finish level to save. Repeat as necessary.
Extra details:
Well first of all you need to be on a dark corners map. They are: Allfathers Garden, Highway, Memorial, Nowhere, Sanctuary War Machine and Way Station .
Second- A BLOODMOUNT is a two legged creature with T-Rex like arms. It is usually ridden by a Grenadiar Elite. For the achievement, only the BLOODMOUNT needs to be torched. In other words, burn the horse, not the rider. Also, a SCORCHER is a famethrower. A flamethrower is a gun that shoots fire. Fire is hot.
Third- The individual kills also do not need to be 100% scorcher. You could shoot a Bloodmount with a gun and then finish them with the scorcher. For example, I could shoot a Bloodmount 3 times with a shotgun and use the scorcher for the final damage. You know you did it right if you get a counter after the kill.
Forth- These can thankfully be done in more than one round and no, it does not matter if you pass or fail the round.
Fifth- Bloodmounts only spawn on levels that end in 0 i.e. 10 20 30 40 50.
Sixth- Never expose Bloodmounts to sunlight, get them wet or feed them after midnight.
My advice if you are going for this specifically would be to set it at casual and choose 10 on a map that always spawns a scorcher like Allfathers Garden. Replace the shotgun with the scorcher. Shoot at a distance with the machine gun until they approach, once they are close roast them and roll back to avoid their pinches. You can let yourself get killed after you reach the end of a round to start again.
If you have someone working with you on this have them start the kill with a machine gun and as the Bloodmounts approach, fry them. Again, you can let yourself get killed if you make it to the end of the round to start again.
MAP LOCATIONS AND OCCURRENCES:
ALLFATHERS GARDEN(BEST)- Flamethrower spawns every round. It is located in the middle of the lower level. It will be under a semi-roofed area. If you are walking along a the perimiter with open sky, you can see it in a U shaped brick area. On the opposite side is a statue with a gernader/torque bow.
Highway- Alternates between scorcher/gernader. It is on the lower level, center of the map. There will be two sets of stairs on the sides and facing the opposite direction are two more staircases that lead to longshots and dead ends. The scorcher is located in an enclosed area with a roof. This little closet looks like where the locusts keep PA gear, a mop and holiday decorations. On the floor above it is an alter table.
Memorial- Alternates gernader/scorcher. Found in the lower level. It is inside the center of the U shaped hallway right in between the sandbags and the stairs to the upper level.
Nowhere- NA....well nowhere.
Sanctuary- NA
Way Station- Alternates scorcher/mulcher. Located on the upper level. You will find it at the end of a runway that is right in line with a stream of lava. There will be two sets of stairs that lead to the runway and you can reach from the top balcony as well.
War Machine- NA
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NA= not available (as a SPAWNING weapon as far as I know)