Just unlocked this trophy and you have to assign all 165 specialization points into the desired tree. I found it hard to get a definite answer as to what is required for this to pop so thought id write this. There is a a node that requires to be level 3 to unlock the next node (at least on Demolitionist this is) Also the best way to get the specialisation points is to complete the daily bounties and also maybe farming the high XP projects and missions as levelling up earns 3 specialisation points.