Since Mega Mines will instantly kill a Night Witch at double its level (A level 6 Mega Mine will kill a level 12 Night Witch), a simple but risky strategy to defend against Night Witches involves placing a Mega Mine in a corner of the map where attackers are likely to place Night Witches (to stack up bats). If the attacker fails to check for traps, the Mega Mine will destroy most if not all of the attacker's Night Witches which will undoubtedly ruin their attack.
Make sure to check for traps when going against the Mega Mine. Use an expendable troop or a high hitpoint troop to activate it first. The Battle Machine can do this job as well; its ability (if unlocked) can regenerate the health it loses from the trap.
You can activate a Mega Mine over a wall by placing a troop in its trigger radius. This can be very effective if your opponent has a Mega Mine inside their base.
Keep in mind that the Mega Mine has a bigger damage radius than trigger radius, allowing it to damage units farther away from the unit that triggered it.
See if your opponent has any empty 2 by 2 space in their base, as it can potentially be a Mega Mine or a Hidden Tesla.
The Mega Mine undergoes significant visual changes at level 3, 5, and 7.
When initially built, the Mega Mine is a large wooden barrel with ropes strapped around it to hold it together. A red band is tied around both ends of the barrel, and tied to the band is a yellow square timer. If the Mega Mine is set to air, chains can be seen suspending the barrel in the air.
At level 2, the ropes are replaced by steel reinforcements that are fixed to the barrel with nails.
At level 3, all of the wooden barrel is painted red.
At level 4, the wooden barrel is replaced by a red metallic sphere with steel parts all over it; there is a band of steel around its circumference, and reinforcements supporting the band, as well as a gray metallic top and nails around it.
At level 5, the square timer disappears, and metal pressure plates appear all over the sphere.
At level 6, metal horns are added to the pressure plates.
At level 7, the horns are removed, and the mine gains a black paint job with a white painted skull, also, there are three instead four reinforcements supporting the band. Pressure plates are gone and it seemed to be slightly rotated clockwise.
At level 8, the horns are added again to the main body.
When set to air mode, the Mega Mine will appear to float, supported by two metal chains.
The Mega Mine does the most damage out of all damaging Builder Base traps.
One Mega Mine and a cluster of four Mines take up the same space and do equal damage at equal levels. However, the mines have a bigger damage radius altogether.
Tapping this icon displays information about the Mega Mine, such as Level, Damage Type and Targets.
Tapping this icon begins upgrading the Mega Mine to the next level, if you have enough resources and a free Master Builder. When the Mega Mine is at maximum level, this icon is not shown.
Tapping this icon instantly upgrades the Mega Mine to the next level, at the cost of one Hammer of Building if you have one. When the Mega Mine is at maximum level, this icon is not shown.
Tapping this icon, which is displayed only while an upgrade is in progress, instantly finishes that upgrade at the cost of the displayed number of Gems.
Tapping this icon, which is displayed only while an upgrade is in progress, instantly finishes that upgrade at the cost of one Book of Building. This icon is only shown if you have at least one Book of Building.