Legion Boltspitter is encountered in the third chapter. To get to it, you need to jump on a couple of shelves.
Legion Boltspitter fights from a distance which means that Strife will be your best choice.
During the battle, the enemy will summon many allies. Destroy weak opponents first - you don't want them running around the arena.
This boss's special attack are fire grenades. They can deal you a lot of damage.
To protect against the attacks, you can hide behind columns and try to approach the boss from behind. You will have a better chance to inflict significant damage.
Kill the boss to continue.