How can I effectively use the Miner?

The miner is one of two ground units that can ignore walls while attacking, the other being the hog. The main differences between hogs and miners being that hogs prioritize defenses, and that miners are invulnerable while underground. That being said, they can be used in a similar way that hogs tend to be used in a GoHoWi attack. They can be used like surgical hogs, in that they can bypass the walls and target specific areas to take out compartments that might be difficult to get to with your main army of golems and wizards. They do have the drawback of not prioritizing defenses, but this can be helpful as you aren't left with buildings around the edges that need to be cleaned up. Also, they as they are underground when moving, they can avoid traps, such as double big bombs, allowing them to traverse areas that would be death traps for hogs.

They can also be used as cleanup troops as they have similar damage per second as wizards and take up similar troop space (5 vs 4), but have the added benefit of significantly more health and the invulnerability while traveling, negating the damage that could be taken while walking to the next building to destroy, as well as avoiding all traps that might kill them. They also will ignore walls, so they will not be stuck attacking walls, which is a significant advantage as high level walls have a large amount of hitpoints.

It should also be noted that while miners are invulnerable to damage while burrowed, they can still be affected by spell. This can be quite helpful, as they can be healed without being damaged by the nearby defenses which will help prevent them from getting focus fired down as they walk between buildings. Also, if they are near your other troops, such as a golem, them going underground will make them lose aggro from the defenses and possibly allow the golem to take aggro and tank for the miners. Also them being able to be healed while underground may make taking out buildings near multitarget infernos easier, as they will be able to be healed when burrowed, even if they are in range of the inferno tower.