Definition: Pokemon that has high attack (Physical, Special, or Both) and can learn high powered moves that counters Tanks and Walls. Consequently Bunker Busters have low defense and/or health. Tow examples are Eboar and Porygon-Z
For Eboar: More of a physical Bunker Buster due to higher ratio of physical attacks that special attacks and Emboar higher higher attack than special attack.
For Porygon-Z: More of a special Bunker Buster due to it's high special attack and can learn various move types to counter most, if not all, combinations of types.
Any questions, comment.
Here my question that I have asked that got all the roles and a 'Bunker Buster' role was not listed in the answer.
These are called wall breakers. They are already a role so sorry about that. They are Pokemon similar to Porygon and Emboar who just smash everything very hard and can "break" walls. Other Examples are Victini with V-create