Here's a strategy which I found to work very well:
Once you take control of the base make sure that you level the Hero with the lord garithos model to at least level 6(I believe that his experience is capped once he reached level 7 in this mission), do this by destroying the creep camps around the area, (btw the murgul alchemist found in the eastern camp gives you a bounty of like 800 gold), make sure to upgrade garithos' Soul siphon to level 3 and also make sure to learn his ultimate combat mastery, this allows you to take a crusader head on. Build lots of cannon towers as well as swordsman/knights/spellcasters to take care of the soldiers.
When the crusaders come take on the first one with the garithos, make sure to cast soul siphon and use Yanes to heal him, also stall the others with entangled roots and other abilities, 1 on 1 a level 6 garithos with combat mastery and soul siphon cast on him should be able to beat a crusader, just make sure he's fighting only one crusader at a time, there's a chain lightening item with several charges that can be obtained by defeating creeps, give this to Yanes and use it on the attackers, with a bit of micro managing this should win the battle for you, you could also build towers in the way to stall them, but the crusaders can literally demolish these within second so it wont be worth the resources.
Also I don't know if its random or not but if you go raid the outpost to the northwest a unit there drop a tome of +2 strength.
Hope that helps.