This guide will let you know where to get the Combustion Beam in Warframe. You have to follow the simple steps in this guide to do so.
The Combustion beams can drop from various corpus enemies, but the ones you’ll be farming are found on corpus ship tilesets on just about any planet. It is recommended to find a survival mission on a ship tileset; in this case, it’s one on Pluto.
In terms of enemies, you will be looking for who can drop the mod. You have to look for Juno sniper crewmen, nullifier crewman, and the different types of comba’s that can spawn on as they all have a chance to drop combustion beam when killed.
It does have a fairly low drop rate of about 1% ish percent, so it’s likely going to take you some time or a few runs before you get the mod as you are going to have to kill a lot of the enemies you’re looking for. The enemies you’re looking for must spawn around the 6-minute mark or after 5 minutes.