First you must turn the power on, to do this travel to the 4th (i think) stop on the bus route, you will find a warehouse type building with a shack next to it. Open the door and drop down. You will be in a bright white corridor, go into the main room and follow the walkway round till you reach a small room. In front of you is a workbench, now you need three components for it because you need to build the power switch, they are:
- A back panel
- A 3 pronged switch
- A zombie hand
The zombie hand is the most difficult to find because it looks exactly like the hands that get littered around when you kill zombies. All three are somewhere within the main white circular room, the entry corridor and the workbench room. To pick them up just walk up to them and there will be an on screen prompt for you (X for 360, square for PS3). You can only carry one part at a time, once you have a part, take it to the workbench and hold X or square to build the part. Once you have built all three, you can then switch on the power, the circular white room will become electrified and a tube will rise with the electrical zombie boss in it, who will then escape! Fun times...
Secondly, you need an EMP grenade from the random box (alternative to the monkey bombs). Now hopefully when the zombie boss attacks he doesn't do it while your on the bus and its when at a stop. When he arrives, avoid him and throw an EMP grenade at him. That should kill him and providing nobody got attacked by him, the achievement should pop for all people in the game.
Hope this guide helps, sorry if it was longwinded, i thought as much detail as possible would be best for all players :)
UPDATE
Credit to Unit215 for confirming that this can be obtained on easy
Credit to x1001x Puppys for discovering when the Avagadro (electrical zombie) spawns, best ending a round while the sky is full of a lightening storm.