Flamentus is a named creature.
Use Flame resistance Gear, pull it out of the monsters to a safe spot, use stun, mezzes and roots. Ice attacks will do substantial damage to Flame related critters. If possible, have Ice attack cast on you, or having the Weapon Tech: Frozen Weapon applied to your weapon will also help immensely (note, Frozen Weapon is a one-time use only tech, so apply it your 'best' weapon). All beetles have a Harden Shell ability granting them higher resistance to almost all damage types for a short duration. Ethereal attacks don't suffer this penalty.
Flamentus has a chance to drop the following: