I really liked this game upgrade system it was very well implemented.
What I didn't like was that enemies were doing like 10x more damage each time the difficulty went up!
I like a challenge but don't like playing level over and over again....
The last boss was mostly annoying than hard.
To Beat Krane:
Stand on the red button to shoot the eye that is currently open.
when he is going to hit the group move fast and get the shield!
That shield is there so that when kraken hits the stage he can not heal!
He heals a lot and would waste all your efforts.
In the last stage wear the mustard shield armor is the best because the shield can keep you alive longer if you have 50% skill recovery.
Then equip Armor 150 hp +20 heal one to keep you alive.
The last stage damage you do does not matter at all since you can't damage kraken only the stage cannon can.Just make sure you have 50% skill recovery for the shield, a lot of Hp and Hp recovery.