This is the culmination of the plants' storyline when you visit Town Center. You must complete 3 achievements prior to this, collecting 3 items from zombie mini bosses and turning your lawnmower into the Mindblower. This will allow you to eliminate the zombot bouncers that block access to certain areas/chests.
From the world start point, head right and enter the sewer system in under the gas station; down the left tunnel you will find the control panel that summons the Mindblower. Drive it in front of the bouncer and detonate with RT. You will be able to enter and face Olds Cool.
Once the battle begins, he will be above you in his DJ booth/disco ball. You can attack him immediately, but watch his counter attacks.
- Missiles will slowly launch from above, shoot them down. (If they connect you will be stunned for a time)
- He will roll his records at you along the ground, you can jump them and move left or right to avoid. (They do decent damage especially if you are stunned) Barriers will also rise out of the ground and these are great for regeneration of health if you need it.
Once you have done enough damage, he will become invincible and his ball will fly into the air and create seismic waves each time he lands, jump over them to avoid damage. He will start spinning after a time and charge you, you need to get him to hit one of the speakers on the wall to stun him and attack the exposed button.
For his final phase he will rotate in the middle of the room with glass panes rotating in parallel lines, these will hurt you upon contact. Destroy these panes until you have a "door" through each section, then attack him while moving through your gaps to avoid damage.
After you have done enough damage he will begin his cycle again. While he was up on above the floor the 2nd time minor zombies attacked, which was good as they gave me a health boost thanks to my Vampiric upgrade.
It took me 2 full cycles to defeat him on my 1st try, I used a Snapdragon with the Vampiric, Speedy and Low Life upgrades.
Good Luck!