Well I've got to admit, I almost always utilise a hand to hand assault Woodelf army, and not having exactly a strong idea of what the rules were for the tornament, the following is the last tournament Woodelf army that I used, (as a bye breaker when I ran MOAB earlier this year in Sydney).
Woodelf General
Armour of Fortune, Shield, Sword of Heroes, Jade Amulet, Longbow
Woodelf Hero
Bow of Loren, Potion of Strength, Light Armour, Shield
Level 2 Woodelf Mage
Destroy Magic Scroll, Skullstaff, Longbow
Level 1 Woodelf Mage
Dispel Scroll, Longbow
Woodelf Wardancer Champion
Additional Hand Weapon and Hail of Doom Arrow
and
4 Wardancers (additional Hand Weapons)
Woodelf Wardancer Champion
Additional Hand Weapon
and
4 Wardancers (additional hand weapons)
6 Scouts (longbows)
6 Waywatchers (longbows)
6 Dryads
2 Treemen
Great Eagle
At first it might appear weak, but it is far from it, and can demolish most armies in hand to hand (all WS 5 or higher bar the Drads and Mages). The shooting is sufficient to soften up the primary 1 or 2 units, and good enough to take out enemy warmachines or to cripple chariots.
I think that this army would stand a good chance of taking out Tim Ray's Skaven or an army of similar design, the main problem would be the Plague Censor Bearers.
I would have the greater mobility by far, as the only unit in formation are the Dryads. Scenery would be most important to combating this force, as being able to "slip through the cracks" would be all important.
Concentrating fire on the Doomwheels and the Plague Censor bearers to ensure that the most of the problem is removed would be essential, but given the Storm Banner being in effect means that you'd want first turn so that you can shoot before the Banner can be activated, and to also shoot before the rats get the chance to close the distance to the hth where their ranks will be all important.
If the rats got first turn, it would be a matter of trying to minimise the damage inflicted by the Doomwheels and PCB's as they would be guarenteed to be leading the charge with the Slaves.
Skaven magic is also quite nasty as many of us could testify too.
The trick, hug the scenery, try and remove Scorch when it pops up, and bide your time. It is important to play careful, controlled and be very disciplined. It would do no good at all to get a full head of steam, only to be destroyed.
Slaves are easily killed, but they are worth little if anypoints. The same goes for the Giant Rats. It could be a game where to win, you need to make it a low scoring affair, alternatively you could attempt to destroy his main unit, but that will undoubtedly be highly defended.
So after all this warbling how would I do it? Well it all really depends upon what the enemy does with his army I suppose. Concentrate on the Doom Wheels first and foremost, disable these as soon as possible with the infiltrating elites and other assorted archers before the Storm Banner gets activated. After these have been disabled, turn full missile attention to the PCB's whilst remaining predominantly hidden, (keeping escape routes for all your units). Get ready in the final rounds to send Treemen and Wardancers to take out a unit or two (both are extremly good for charging on the last turn if you went first as they are likely to survive a countercharge if it comes - Tough/Shadows coil).
But I suppose one thing to remember is that the Storm Banner probably won't last the entire game. Both players roll for it to wear out IIRC similar to the Black Gem.
Anyway, I agree that it would be hard, but no harder then many other armies lead by a competent general.
I wouldn't change my army list, or design for one magic item, especially an item that might last only a single turn.