I would suggest putting Nimrook into your actual team and do what you would do with any underlevelled character; plenty of passes, use techs, try and get him into encounters to try and intercept the ball / steal the ball through tackles, since those actions give a lot of XP. If you happen to have Hi-Risk (I don't think Nimrook can learn it, but you can use Tech Find to unlock it), that will shorten the process. But is there really a point? If nobody gets past your defenders, Nimrook could have 0 CA and it wouldn't make a difference.