Press escape (This should open up the pause menu)

Go to the "Logs" tab
Click on the "Primary Mission" section of the screen
Scroll down to "Getting the Band Back Together" mission
Click on the mission name on the left side of the screen
Click on the button labeled "Replay"

This should start up the mission and allow you to replay it.

You'll spawn at the Las Catrinas hospital. Once you get your waypoint at the Marina, head towards it.

Once you get to the waypoint, talk to Wendy.
Finish up this part of the mission by doing what the game tells you to do. Harpoon Wendy's boat over to the empty dock, protect the harpoons as they slowly reel Wendy in, and clear out the OD so she can get out.

Once you've finished the first part of the mission, go to the waypoint at the Concert Hall and talk to 4Kim.
After talking to 4Kim, get down to the truck that's marked with a waypoint and interact with it to get to the drums.
Once the cut scene is finished, head over to the Taiko Drums that have been dispersed around the area. DON'T FINISH THIS PART OF THE MISSION.

Do what this step is called and just unload upon the never ending flow of OD. Use every weapon on you and you'll pretty quickly level up each weapon. Not to mention racking up a combo and getting your style points maxed out is a breeze.
Quick tip:

give yourself the personal space amp, this will allow you to bounce around with less of a chance of being batted off of your chosen bounce pad (be it an umbrella, the nearby police car, hell, even one of the Taiko drums)

Hopefully this will save you some time, as it took me a while to find a solution for myself. Have fun killin' them OD!