Flemoids have no contact damage; They only damage you when they are attacking. So feel free to rub up against them; It won't matter when you consider the next tip...

Zorch attacks don't just deal damage; They also stun the flemoids for their damage animation. This pairs with the above tip; As long as you're shooting with enough firerate, you stunlock the flemoids and can faceroll them for no threat.

When I refer to weapons, I'll usually refer to the doom weapon that they replaced in the original. https://chexquest.fandom.com/wiki/Zorcher

I'll see if I can get decent damage stats on the flemoids. A couple of things I know out of the gate though:

Flemoidus Communus takes 3 shots with the pistol or chaingun. I need to double-check, but since they use the same ammo, I'm assuming the same damage and just a change in fire rate.

Flemoidus Bipedicus takes 5 shots with the pistol or chaingun.

Both take a single shot with the Large Zorcher/Shotgun

Flemoidus Cycloptis can be taken down with two shotgun blasts.

I have noticed that sometimes I get extra damage while strafing and firing; I believe the shot is clipping the hitbox in a way that multiplies the damage, but I can't reliably recreate it. A good example occurs around ~0:37 in my NewGame Run, where two shots take out a Communus: Also at ~0:45 with another Communus. Compare with the first in the game taking 3 shots at ~0:20

I'll add in more stuff as I find it, but a couple of observations that I thought would be nice to share.