PileOfJunkKind

Once combat starts, roll a 1d4 and a 3+1d5. This is your durability and the number of faces on your damage die. After 1d4 successful attacks, the will break, and you expend a major action to get another glob of junk. (Roll another 1d4 and 3+1d3)

Here's an example.

Roger has a Junkkind specibus. He rolls a 2 for durability, and a 5 for hit dice. During a fight, he can successfully attack 2 times before the paper ceases to function, and rolls 1d5 for damage. After he strikes an imp 2 times successfully, Roger must expend a major action to get another glob of junk, rolling the 1d4. He gets 3 durability. and 1d8 damage die. The next round, he defeats the enemy encounter. The next combat encounter that happens, he must roll durability and hit dice again.