How do you get past helms deep... it's impossible !?

Helms deep is hard especially when the beserker's use staggerfoe.

If you have okay lightcraft aura of life can help
On the ladder bit just take out the warriors then the ladder behind them the last few bits are the hardest
Berethors Leadership might help depending on how good it is.

If Idrial can't use spells her wounding strike is quite good

Well it depends where you are. First when you're fighting with Legolas, I suggest using his Lorien Wrath all the time. Concentrate on one enemy at a time. After that oaf is down, be sure to take out the ladder behind him as soon as possible. That way no more Uruks will come to bother you.

Also I recommend Berethor's Leadership, Stand Fast (immunity to stunning). That way when the Berserkers go berserk and unleash Stagger Foe, you'll have your players immune. Best combination - Legolas - Berethor - Idrial - Hadhod. Morwen's attacks are good, but she dies within two normal strikes and one Critical hit. Elegost is around the same par as Morwen, and Eaoden is slightly better than the said two.

Then if you're aiding Aragorn at the Gate, keep using his Bane of Sauron. If you don't have enough AP just use Lembas, Leaves of Lembas or Sachel of Lembas. Hadhod's Mountain Rage is good too, if you've gotten it (if you play the Hard Mode, you'll get it in no time. But in the Easy mode you'll only get it [unless you use him in each battle] when you reach Sauron). Best Combination - Aragorn - Berethor - Hadhod - Elegost/Idrial/Eaoden. (Idrial for her healing; otherwise she's not much use because the enemy's dexterity is very high [i've made her dexterity the highest it can go and still she misses] but her Elven Rage (or Gil-Galad Rampage; you'll only be able to get it here if you play Hard Mode) is pretty good, aside from the fact that she misses).

After that it's with Gimli. Using Wrath of Moria or Balin... Balin something (sorry), is pretty good. Whenever you're with the heroes, it's not really that hard. Use Idrial's Haste of the Elves on Gimli. Works like a charm. Berethor's Ecthelion Wrath or Citadel Rage, or Uruk-Hai Bane are good. Hadhod's Mountain Rage, or Hew Armour, makes him worthy. Eaoden's Rupture Armour is nice, and Morwen's Rage of the Maiden is good, though I don't think you'll be wanting to use her since she dies in two hits. Elegost's Aimed Shot guarantees that he won't miss, but his Arrows of Sleep (if you've got it) work like magic. If you don't have it, try Pierce Will and Piercing Might. Idrial... is good for healing, but as i've said time and again, Elven Rage and Gil-Galad Rampage are good; if not, try Stunning and Wounding Strike.

Then last if it's when that lousy Berethor utters his lousy words "i do not fear them... i fear no one" battle, i suggest Berethor - Idrial - Hadhod, but switch between Hadhod and Eaoden. Keep hitting the first troll first, and remember to use Berethor's Stand Fast. After that, hit the other three Berserkers with Berethor's Ecthelion Wrath (if you have it) or Citadel Rage (you should have it by now) if not then Uruk-Hai Bane. Then Hadhod's Mountain Rage (if you have it too) or else his Hew Armour to lower their armour. If you're using Eaoden use his Rupture Armour. It's brilliant, but unfortunately his armour values are too low. Berethor's Targeted Strike doesn't seem to do much good here though.

Try and use Fellowship Grace and Royal Grace if you have them, cause you're going to be spending tons of AP and losing loads of HP here. After you're done with the three Berserkers the rest of the fight should be a load easier. Also, when i played in Easy Mode Perfect Mode came once, Normal Mode once or twice, and Hard Mode, three times. Don't stinge on Perfect Mode; use it as quickly as can be. That way it'll be much easier too. I highly recommend Fangorn Bash, or it not then Roots of Wrath. Try and choose the hits that injure multiple targets, not just one.

There're three trolls in Easy and Normal mode, i think, and four in Hard. I THINK, i'm not too sure. Though these oafs are so large, they're rather stupid. Berserkers are more to be reckoned with, so be sure to not stinge on AP when targeting them. Idrial is only there for her healing. You can change her, if you're ready to see the Game Over screen soon. When she's not needed for healing her Elven Rage (if you've gotten if) is good. Gil-Galad Rampage is even more so, but you can only get it so early if you play Hard Mode and use her in every battle.

It was hell for me the first time, but after that, they were strong breezes against my body.

The main keys to your success is having one character learn "Power of Valar" with lightcraft. Also using Idial's grey Haven and her spirit powers is important. Also, take note the trolls can be paralyzed (Disabling their melee) by Elegost's paralyzing shot. That will take them out of the battle completely. Also, the urk mages, you can use a shadowcraft move called "Silence" then they can only do a week melee attack. Finally, berethor and his "Stand Fast" under leadership will make your party immune to stuns by the trolls and default Urks. Good luck!