two devastating uses of hellhounds i have witnessed and used.
#1 hellhound+rough riders. get your rough riders behind any unit, and your HH in front, shoot with all the rough riders, then with the inferno cannon, causing the unit to fall back into the riders, causing crossfire.
#2 HH+sentinels with heavy flamers. pretty much the same as witht the rough riders.
#3 two hellhounds. the same as above, but incase they pass one test, they have too take another.
#4 hellhound + ogryns. as the enemy get near, get the HH behind them, cause casualties, then assaukt with ogryns, maybe kill them and consolidate into others.
heres something i tried once(i was using a lot of a friends IG army). leman russ/exterminator/demolisher+chimera full of ogryns+hellhound+storm troopers.
advance the field using the big tank o shield the smaller ones, deep strike the storm troopers near their part of the field but dont do anything else with them, save them. once the tanks arrive, get the chimera and HH out from behind the big tank, fire ordinancce, drop the ogryns by the storm troopers, and you have yourself a table quarter.