It pretty much is horde mode, with a "grab the bomb and stick it on the tank" twist every few rounds.
Pretty enjoyable but can't imagine what the challenge mode version will be like. It's buggy as hell though, cost us a few restarts and locked us out of the game for 5 minutes at one point.
Once you work out the strategy it is pretty easy really. Thoroughly enjoyed it if I'm honest, bugs aside.
Good fun for the run, it's was just a case of learning what each wave brought, and how to deal with it
. The bugs were a right pain, but at least it should get easy now with each run.At least in challange mode and in the DZ, it's raining yellows

*. Second time I didn't fall through the floor but the boss didn't drop the Shade tech so we had to wipe (harder to achieve than it seems - I would often survive with a sliver of health...) and re-do the end again. 