Question
Do mops regen hp while you're attacking them or in between i.e. you attack venz and come back is it at full hp or has it regened some hp?
Answer
Yes. An NPC will regenerate HP in much the same way a PC will. The regen rate depends very much on the mop in question. In some cases, mops will regen HP faster than you can deal damage.

Most monsters you fight in
Most monsters you fight in normal every day combat are not going to regen an amount that will be noticeable before you kill them.
Bosses regen far more than any other monster. Some bosses can regen upwards of 100hp a second.
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Like the stupid DOP boss??
Like the stupid DOP boss?? Just an insane amount of regen.
I think dragons have a high
I think dragons have a high regen aswell.. if you cant out damage its regen u cant kill it
Thats which one I was
Thats which one I was referring to above. Dragons have about 500hp regen every 5sec.
If it werent for regen, all bosses would be so much easier. To easy. Including dragons.
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When/why do mops stop
When/why do mops stop attcking you and go after pets/summons?
Generally a monster goes for
Generally a monster goes for the attacker that is doing the most damage to it. If your summons are outdamaging you, which is the case in most mage situations... Then the monster will turn and kill your summon/pets.
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cause ur pet/summons prob
cause ur pet/summons prob did more dmge then you or cause u walked out of thier screen range and ur mops were the first thing they would attack after u have left thier vision