have a variable that acts like the boss "stamina" so whenever he moving and is "rested" or has enough "stamina" he can walk, during the walk, he will naturally lose "Stamina" like in real life, you get tired after running/walking etc, especially if you have like 3 metric tons of steel on you.
so variable called it Stamina.
Variable call it Rested.
and we can do something like this pseudo-code:
if Stamina > 0
action > subtract from Stamina 0.01 ( will take around 10-12 seconds to deplete it )
If Rested = 1
action > enable boss.bulletbehavior
action > boss.set bullet.speedTo Stamina *5
If Stamina <= 0
action > disable.boss.bulletbehavior
action > set Rested = 2 (he is not rested now)
Now we regenerate the stamina while the Boss is out of stamina.
If Rested = 2 &
If Stamina < 100
action > add to Stamina 0.01 (it will take around 10-12 seconds to fully restore stamina to value of 100)
triggeronce subevent action > boss.playanimation "tired"
Now we make the fat Boss run again when stamina is full again
If Rested = 2 &
If Stamina >= 100
action > set Rested = 1
and the stamina and movement of boss + regeneration of stamina, should be in a loop now until you manage to leave him with zero health or something that "keals him"
hope it makes any sense or helps at all.
edited the animation setup (needed be on a trigger subevent)
here is StaminaBasedMovement example based on the above logic, with some extra stuff inside.