Hi,
Welcome to Dakka
If a model is in close combat you would always use
WS unless its otherwise specified in the units rules (
IIRC I haven't seen that ever)
Any unit charging is always
2D6 however there are rules which allow re-rolling of the dice (fleet etc) so they may be shortcutting the reroll by just rolling three dice and taking the highest.
I've not played
DV so I can't see if the mission is fair or not, is there terrain on the table for the helbrute to get behind for a cover save? that might save it for a turn or two.