Jack be Nimble is a location-based challenge in Shock Drop Slaughter Pit. To perform this challenge in Abandoned Training Facility, the Vault Hunters must jump over the lava. It is worth 5 Badass Rank.
There are many points where this can be easily achieved. Even with out boost from an Oz kit, raised platform and buildings on the southern side of the lava pit provide several launch points to complete the challenge. Even walking over a small metal catwalk at the far western edge of the pit will complete the challenge.