Putting "Assassin" on the Banshee mask would mean it would only operate in B-B (ie in CC). Effectively it represents the power sword capabilities of slicing up enemy infantry. If applied I would recommend reverting to CC3+. Note however against Terminator armour there is ~50:50 chance of killing in First Strike, so you would need to double up. Doing this should leave 1-2 Termies alive, causing 1-3 HB casualties in return and leaving the HBs up in the assault resolution.
Changing their mobility to 20cm with infiltrator would mean they could be positioned 31cm-40cm from the Terminator target (so outside shooting and assault range), when the terminators have a reasonable chance of survival if they get the strategy roll and just move elsewhere. OTOH, if the HBs get to go first, the termies should be toast
