After spending way too much time walking back and forth between zones I found a way that might be useful to those who are looking to obtain the Yagyuu Darkblade as quick as possible. There might be a better way but this was as least painful for me and upon re-entering the zone the Bombshell rare monster was up more times than not.
The general idea is to chain the Bombshell and to get him to repop as fast as possible. Here's what I did:
Once you're in Lhusu Mines and in the Lasche Span the fastest way to get him to pop is to take the south zone and take the cart to Site 11, immediately re-enter the cart and take it back to Site 5. Zone into Lasche span and there's a good chance he'll be up (even after killing him and taking the cart rides). If he's not you can either zone back and forth a few times then walk the span or zone by crossing the span each time. Doing this, the most times I had to zone the span was 3 times.
My party makeup was a tank,dd, and support/mage. Setting up the party the following way allowed me to just enter and engage without much effort:
Tank: Ensanquined shield, Attack
DD: Fastest weapon avaiable, cat ear hood/genji gloves
Mage: Gambits - Decoy tank, berserk/haste/bravery DD, dispel foe status protect, cure.
This should allow the DD to make quick work of the bomb while the tank blocks most (if not all) of the bomb's attack with the ensanguined shield.
After the fight, remove the shield, dispel the DD's berserk (so he doesn't automatically kill the bats when trying to enter the carts) and take the cart rides back and forth as mentioned above. It will still take plenty of kills before the bomb drops the katana but this was the least painful way for me to do it.
Hope it helps!