This character has skills from the “demolition” mastery. In the screenshots you can see two skills currently named “canister bomb” and “thermite mines”. Also, as a main attack the character is using “ammo modification” with “explosive rounds”.
For anyone who is curious, thermite is an extremely hot burning combination of a metal powder and an oxidizer. The reaction was first discovered in the late 1800s. I figured that was close enough that we could co-opt it for the game. 
The mines work well for creating a barrier that will kill hordes of weak enemies or wear down slow-moving armored enemies. Naturally, they’re not as effective vs. ranged enemies who can just fire over them or destroy them from a distance or fast-moving enemies who won’t take as much damage passing through them.
The canister bomb is lobbed like a grenade into the middle of a group of enemies where it explodes and sends out several fragments that explode over a wide area. It is great for taking out hordes of zombies, grobles, or the like. It isn’t as effective against tougher enemies like the dranghoul.
The grobles have a shaman who can chain-heal and if there is more than one in a group, it is often difficult to kill anything until you dispatch the shaman. For this job the demolitionist can use either a flashbang to disrupt and confuse the shaman or a grenado to knock them down / potentially kill them.
The grenado is more of a focused explosion good again tougher single targets or tightly clustered groups. It is fun to lob over walls to kill enemies in a house before you enter, although, if the roof is intact, then you can drop some thermite mines are the door before you break it down.