Eldar troops are very specialised, so they will probably produce lots and lots of warp spiders and dark reapers against your infantry. You need lots of leman russes. And grenade launchers are better than plasma guns, i think.
I kinda forgot how good IG grenade launchers are, i spent some time outfitting them with plasma just to get used to using it and got away from GLS. I also forgot how good basalisks are, especially if used right. I have adopted a more patient and micro style(this was hard as I am very impatient and pretty lazy) and am repwing with IG. I have also realized that putting inf and my commander squad in chimeras and sneaking them into your enemies base to kill their HQ while your enemies are throwing himself agianst your basically impeneratrable defense is a really good tactic.
I once had my stronghold attacked by Necrons, and with no reinforcements, though I did win. Same with Orks (I've had my stronghold attacked twice, when playing them). - Won both times. What I tend to do, is just rush their HQ. You've got a vague idea of where it would be, so get a few Sentinels and run along, blow it up. Good Luck!
Sentinals are nice. Not tough, but they hit hard and a relatively cheap.
I didn't make it!
I agree with Ash, the best method is to just rush their base before they get a chance to build up a force, I had some trouble taking the Bulwark from the orks while i was using necrons, The problem was i tried building my force before attacking them but they just overwhelmed me and blew me to bits even with the monolith up and running, So after a few attempts i just got my trainers on and went for a jog straight to their first base along with my honor guard and owned them, beat them in less than 10 minutes.