For Ready for deployment, my main account kazimodo99 was already at level 14. So what I did was to set up a combat training session with 2 Controllers split screen and with those settings (Time limits : 30 min, Score limits : 30 000 pts, 2 friends against 9 ennemies, 2nd class difficulty).

So the game starts. At first, I'm using my regular account player/controller 1 named kazimodo99 and walk around killing people until I levelled up. During that time, since I was playing solo, the 2nd controller/player was idle, not moving. As soon as I've levelled up (now to level 15), I changed controller (now playing with kazimodo(2) ). Since it is a copy of your initial account he is on level 14. So basically, I start killing AI and levelled up to 15 as well.

And Bingo, as soon as I reached level 15 with my second controller.. the trophy popped. Brilliant.. absolutely brilliant

Here is the solution! Guaranteed to work