The Ol' Tank

About
The Ol' Tank is the fourth boss encountered in the game, and considered to be relatively hard, especially for slower characters like Redd and Crimson because they tend to fall behind his curve.

Behavior
Normal Mode: The boss slowly moves around the center of the screen in a circle and rotates around spraying bullets at the player. As he takes damage he unlocks more and more attacks to use against the player.

Always: Sprays bullets are the player.

Below 4000 health: Shoots longer, faster, more damaging bullets in a circle, shooting one every 30 degrees (30 frames).

Below 3000 health: Shoots more of the same bullets as above, but these are always aimed directly at the player. This makes for a situation where the player is crushed between both volleys every so often.

Below 2000 health: Fires a tracer bullet every 150 frames that slowly chases the player and dies off after 300 frames.

Below 1000 health: Widens his basic attack into a huge cone and shoots faster.

Hard Mode: His basic bullets deal extra damage and his tracer bullets pursue the player faster.

Strategy
The best way to defeat this boss is to circle around him constantly, keeping him under constant fire. Faster characters are better for this because slower characters tend to be unable to keep up with the boss's rotation and take more hits. Once he gets below 3000 health this becomes a bit harder because every know and then you will start to get crushed by his sniper bullets. When this happens try to change direction and only get hit by one sniper bullet. Once the boss is below 1000 health you will really have to start dodging a lot until he's dead. Using Redd's mega laser can help you quickly end this stage.