Mad Bomber is an encounter inside the The Barrens set of Mercenary Bounties.
This bounty can be picked up from the Bounty Board and recommends a Party level of 15 to complete it.
You can unlock Mad Bomber by first completing Barak Kodobane.
Completing this Bounty will unlock the Plaguemaw the Rotting Heroic Bounty.
Potential Mad Bomber Bounty Rewards
In addition to earning Mercenary Coins based on your Party members, Bounties are assigned to specific Mercenaries to allow you to target loot better.
The following Mercenaries can have their loot appear in the Mad Bomber Bounty: