Make sure to setup a regular mythicmob with your animation from start to death will be the entire animation.
When making a mob, make sure to also add a skill that will remove the mob after the correct time too. Or else the animation will persist eternally and cause lag. For instance in this death animation "death-spike" i have the anim-death-spike skill present.
That skill will delete the mob after a certain amount of ticks that works with my animation.
This means after 85 ticks (4.25 seconds) the mythicmob will dissapear preventing lag.
This example also integrates ModelEngine via Mythicmobs for a 3D custom model animation with a Resource Pack.