Hey, thanks for the positive feedback, glad you liked the game!
We definitely could have explained it better, but each enemy has two different stats for defending against the player's attacks, one for the normal attack, and one for the special attack. Each different kaiju has a different amount for each, so it's up to the player to decide which attack type they want to upgrade more. We did this to add a little more depth to the battle system without sacrificing too much time.
Upgrading your health more can be good way to be able to last longer, but a block gauge or armor system is a great suggestion, thanks!