Bloodmounts are only found at the end of the last wave of each sequence in Horde mode. Each sequence consists of 10 waves, so waves ending in 0 are what you'll need. Throw it on Casual to make it easier on yourself. You can kill a few then let the other Locust kill you and retry the wave, your stats will be saved. A Scorcher can be found on the following Dark Corners maps:
- Allfathers Garden
- Way Station
The Scorcher on Allfathers Garden is always present, but the other three maps it rotates with another weapon every round. If you don't want to be dying twice to get back to the "on" round for the Scorcher, just use AG for this achievement.