Transformers: The Game

Before attempting this achievement I had read that locating Optimus Prime is the hardest part, (which is true), and that his locations are random, (this was not true for me).

Here are the locations (all directions are based on my assumption that the construction yard is in the SW)

Location 1: Where the chapter starts.

Location 2: Follow the road running up the West side of the park, just to the North of the Park's NW corner. (From Location 1 head up the hill to the park and then turn left, he's dead ahead)

Location 3: SE corner of the map, directly opposite to the construction yard. (From Location 2 follow the road South down the West edge of the park all the way to the Construction yard at the end of the map, turn left and he's dead ahead)
*Also a great place to get the Evil combo, after fighting the Bulky Drone and Twirly Crane Drone for a bit Optimus will run back to the corner...if you hit him he won't block and you can get the combo with ease

Location 4: On the patio area with the blue and white umbrellas just North West of the construction yard, SW corner of the map. (From Location 3 head West to the construction yard, turn right and then first left, following the road around to Optimus.

Location 5: On the road that runs accross the North edge of the park, just West of the NW corner. Should be the road North of the Helio Phone shop. (From Location 4 head East 2 blocks to the SW corner of the Park, turn left and head North 2 blocks to the NW corner, then turn left again and there he is...last encounter for chapter completion).

I had to do this twice as I narrowly missed the final location and all 5 were exactly the same...hopefully this will save people some time.

Complete the final battle against Optimus Prime as Megatron to unlock this achievement.

The fight itself is not that difficult considering it's the final battle, the only real problem is locating Optimus Prime after he runs away and you have finished with the other Autobots.

Don't forget to collect you skills, if you are going for them.

A great solution to obtaining the Evil skill was put forward by WarriorPriest13. - After Optimus runs away for the first time, 2 green Autobots will spawn, target them and fire leaving a small time gap between each shot using you main weapon (not punch). You may need to quickly turn and target the 2nd green one to get all 5 to ping, this mehtod appears to be the only way to get it, and it worked a treat for me.