After transforming into its upright position, the health the monster initially recovers is substantially larger than the standard Reaver. The initial recovery is high, that normally the Reaver reverts back to full health. The health regeneration perk the monster gains is somewhat more potent. Forcing players to play very aggressively in order to defeat this monster. The weapon it drops: The Flesh Render, provides the player with a regen buff at the beginning of battle that lasts for between 15 seconds and 30 seconds (achieving a 30 second regen buff from the weapon requires the player to upgrade it via the Forge in Hard Mode).
A good tactic for defeating the monster, is to utilize the Angel of Valhalla (preferably at its highest stage of growth) as well as ice and thunder magic. Particularly of the -ra variety. Also, due to its heavy regeneration after being upright, it is advised to use overclock after the first stagger when it's upright and continue to use it until the monster has fallen back down from a Launch, or until the player's EP is used up.
Another good tactic is to simply spam overclocks and use the hardest hitting physical schema at the start of battle, after a stagger. If done correctly, the player may be able to take out the monster before it even transforms to the upright position.