Their DEF is 100%, but normal attacks will lower DEF. If you don't have Levia, or Nartas like me, you have to use mercs with multiple normal attacks to swiftly lower their DEF.

☆☆☆ Underground 1F ☆☆☆

1. ★6 Isabel+15
2. ★6 BDM N-0524+9 (Kill 3|2|4)
3. ★6 Veronia+12
4. ★6 Kry+15 (Kill 7|1|9)
5. ★6 Dr. Morgan+9
6. ★6 Leto+10 (Kill 6|5|8)

☆☆☆ Underground 2F ☆☆☆

Among this mechanic, I thought this floor is the most difficult. My strategy here is completely relaying on Granhildr's reflect. Granhildr, and Refithea may have to be +15.

1. ★5 Rogan+1
2. ★6 Corette+10 (Kill 7, 6)
3. ★6 Refithea+15
4. ★6 Veronia+12
5. ★6 Granhildr+15
6. ★6 Christina+15

☆☆☆ Underground 3F ☆☆☆

1. ★6 Michaela+15
2. ★6 Sarubia+10
3. ★6 Christina+15
4. ★6 Kaoli+12

☆☆☆ Underground 4F ☆☆☆

1. ★6 Serendia+15
2. ★6 Naressa+10
3. ★6 Michaela+15
4. ★6 Christina+15 (Kill 3, 4, 5|7, 8, 9|1)
5. ★6 Niya+15 (Kill 2|6)

☆☆☆ Underground 5F ☆☆☆

This one also relaying on Granhildr's reflect. Even with Granhildr+15, and Refithea+15, sometimes won't be able to kill mobs and that will result time over. So you might have to try few times.

1. ★6 Refithea+15
2. ★6 Veronia+12
3. ★6 Lyudmila+13 (Kill 5|4)
4. ★6 BDM N-0524+9
5. ★6 Granhildr+15