It's because of his garbage crit and physical resistance. Unless you've got guaranteed crits or can deal some energy damage, damage against him sucks after the first minute or so of the fight. It sucks taking all 5 minutes of the fight just to do 3-4% despite everything else going perfectly.
With Hit-monkey's guaranteed crits you will have gentle's reinforce passive up at all times. You can see it in your screenshot, the blue icon in the second row of effects. That makes it so you cannot crit, at all, no matter what crit rate or guaranteed crits you have. So then with his physical resist it takes you down to almost no damage. Gentle is a huge pain with this and his short windows to counter his heavy and specials. Since most good raid attackers do their damage though massive crits.
Don't let him to launch sp2 just sp1, you still no critic but on this way gentle won't gain his phisical resistence, which make him more annoyed, happened to me 1 month ago, I go with NS with prowess node and I barely make damage.
Gentle’s reinforce passive shuts down hit monkey’s entire kit because of the guaranteed crits.
I know HM isn’t the best option and his kit can get shut down, but one fight after getting a charge I was able to do decent damage, 50k attacks, every other time even after getting the charge it was putting up tiny numbers like this, the entire fight, every attack.