Something else that is of a huge aid to defeating those Hydras and other nasty monsters such as manticores and dragons is the use of DoW cannons. Though GW has officially closed down the Dogs of War list as a viable army to be taken to the field. In the group that I run here we had an 'officers' meeting in which we decided to continoue to allow the use of DoW. So if your group allows this that's great! If not perhaps you could work a little diplomatic magic in there and get them allowed!!
Regarding corsairs. They have a sea dragon coat which gives them a +2 save vs shooting and spell attacks (and +1 vs close combat) - which makes them very hard to whittle down with blowpipes if you're playing smaller games using just skirmishers A trick I've heard about assassins/shades, is deploying out of line of sight - reveal the assassin, which displaces one shade model... and using the 1" continuity rule for skirmishers place the displaced shade model in los for a charge.
Another thought I had for these threads... if there are people familiar with lists used at various point levels, to post the type and scale of units you'd find at these levels, so that way people can have an idea when to expect certain types of units. (Example for lizardmen, a skink chief is a decent general at sub-1000 point games (usually mounted on a terradon), but past that you'll probably only see him mounted on a steg with a war-spear)