Hi, thank you for your feedback!
The third room is where the infamous "gunner", or "cannon" enemies start to appear. They will shoot you from very far away. And the explosion is the deadliest attack. I should make the shots much slower so that the player can see them coming more easily! :)
Their "weakness" is that they cannot move. But they have a close range pulse shield that keeps anyone far from it, so you cannot attack them when their shield is active.
Maybe try to upgrade your pet more using the loots from the previous rooms. Increasing the range (from you) at which the pet can attack can help you a lot. Also you can increase its attack speed and its damage. :)