This nicely demonstrated the main weakness of the HBs:- when performing an assault they need to avoid situations where there is a lot of support fire, because their weaker armour usually means they get destroyed. The problem with having to go into CC with a target is that this requires the target formation to be at least 15cm from its neighbouring friends to avoid this supporting fire, so fairly isolated and ripe for any assault. Alternatively they need to completely destroy the target to prevent supporting fire, requiring the target to be relatively small and weak.
Here, they target the remaining 2x Baneblades from a super-heavy support company that is right next to the Regimental infantry HQ company. This amounts to 6x DC of 4+RA, which statistically will require some 24 hits to assure its destruction - though it is reasonable to assume at least one critical as well. At best the HBs and Vampire can generate 12 hits including a MW hit, so might reasonably expect a maximum of 3 DC damage which is not enough to destroy both Baneblades unless they both crit. This in turn almost guarantees the support fire from those units of the Reg infantry HQ that can see past any surviving WE.
As the HBs are nearly always played very aggressively in your games, I am slightly surprised that they were not positioned to attack both Baneblades, attempting to crit both WE (at the risk of self destruction
). As it was, attacking a target that had just removed all its BMs was never going to be easy, and given the poor dice rolling, doing so through AA fire (thus starting the assault resolution -2) was very *courageous*. 
Note, if the Vampire had been positioned touching the target Baneblade with all the HBs behind it and the Vampire (relative to the infantry company), this would prevent the second Baneblade from targeting the HBs and a large part of the support fire might well have been blocked by that Baneblade and the Vampire (had it survived). Against a prepped target, this might have given the HBs a slim chance of winning and consolidating beyond the ruins, outside the assault range of the Reg Infantry HQ. . . .
so it is very likely that 3-4 units will be firefighting when you assault. Why not try out Warp spiders if you want the first strike feel or even better some Fire dragons or Avengers if you use them as "ordinary" assault units. It is also very risky to have the Exarch in a CC unit so they might be killed and the Inspiring bonus is lost.
