I added our game public repo to the submission, you should be able to view it, if any problems occur - PM us! :D 

Death animations are simple. We just made some animations of cacti exploding, and then forced Animated Sprite to play them, when enemy health drops to 0:

func attacked(damage, knockback_dir = 0):
    health -= damage
    if health <= 0:
        #sum other code
        is_dead = true
        $AnimatedSprite.play("Death")

It goes the same for Terrorist. Runner, on the other hand, changes his rotation and velocity to simulate  tripping over:

func _physics_process(delta):
    #runner mechanics
    if is_dead && rotation_degrees < 90: #if runner is dead, and hasn't hit the ground yet, make him rotate
        speed = 30
        rotation_degrees += 5
    if rotation_degrees >= 90: #make runner move slower as he trips over
        if velocity.x > 0:
            velocity.x -= 1
        else:
            velocity.x += 1
func attacked(damage, knockback_dir = 0):
    health -= damage
    if health <= 0:
        is_dead = true
        $AnimatedSprite.play("Death")

The Boss just disables his collision, since he doesn't have any death animation:

func attacked(damage, knockback_dir = 0):
    health -= damage
    if health <= 0:
        is_dead = true
        set_collision_mask(0)