If you can perfect guard his four-hit combo. That's not an easy thing to do, and one slip-up knocks you down to an A-rank for damage. The game should absolutely give huge bonuses for risky perfect guards.
Pguarding a hardmode elite boar (pre-nerf) repeatedly is harder than Pguarding Marino's combo consistently. I guarantee you that
no melee player would be able to handle Marino as is without learning to Pguard the combo (or frosty-cheesing). Simultaneously, once a melee player learns to Pguard the entire combo, they won't need
all blizzards to go away while they deal
x2 damage. A simple fix to the "Pguarding difficulty" is to employ quick reflexes. It's something that beginning and intermediate melee players really should employ around FF/Seasonne because melee's reliance on Pguarding picks up
significantly around that time.
Additionally, guarding the first attack is easy. So as long as a player is careful to guard at least somewhat near the right times, they won't take damage. However, they will miss out on the stunning opportunity if their timing isn't good enough.
If melee doesn't get an easy OP reward for Pguarding an elite boar or elite bot, I imagine one won't come during a boss battle. That reminds me that a nerf needs to be applied to empowered autumn knights though.
The "cross-blizzard nerf" that ensures 1-2 squares of safety for the player without any damage multipliers is most fair imo. It prevents Marino from being shifted from a mage-dominating boss to a melee-dominating boss. And it protects both sides (95-99.9% of the time) from running into jank blizzard situations if they Pguard the entire combo.