The fastest way to get this - if you have a second controller - is to use split-screen mode.
Play as Citron, start split screen mode (in the plants area, next to the customisation chamber) and have the second controller guest play as an imp.
Send Citron down to the sewer area in the backyard battleground. If you enter from the area just to the side of the flag, you'll go down, turn right, and then see a stall area. Send the imp down here also.
You can then summon the mech using the imp when ready. Use Citron's EMP, and then suicide off the path into the green sewer muck. Respawn as soon as you can - both the respawn locations are very close by and your EMP will be fully recharged, so repeat the process until the Mech battery runs out. Wait for the Imp's mech summon ability to recharge, and go again.
I found I was normally getting 5-6 EMP shots on the Mech each time, but the trophy popped a lot sooner than I anticipated - so it's possible some of these were counting as multiple hits. It took me around 15 minutes.