Here’s a pretty easy way to farm OD kills and level up each weapon you own. (This will also help unlock the achievement “I Like ‘Em All)
Step One: Start up the mission “Getting the Band Back Together”
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.
Step 2: Get Through the First Part of the Mission
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Once you get to the waypoint, talk to Wendy.
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Step 3: Get to 4Kim
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Step 4: KILL EM ALL!!!
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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!