Elven nobles are quite potent characters. Thanks to Vaul, we have a nice selection of magical items with which we can make our heroes rival some lord-level characters.

Thanks to Speed of Asuryan, almost all elven nobles carry a great weapon, unless they are mounted, in which case they boast a lance. this choice of weaponry might seem a bit dull, and even nonsensical when you think about the loss of a shield when fighting on foot. But here's where the magic item allowance kicks in. The Dragon armour of Caledor is a veritable steal, and a great way to give your haughty noble some much needed protection. Couple it with the Guardian Phoenix and you've got almost too much protection. There are of course other combinations, and if you mount your lord, you can get a 2+, or even a 1+ armour save, that is also rerollable, and still retain that ward save from the Guardian Phoenix.

While the deadly yet survivable elf might be a classic, we have other, more exotic. options available. My personal favourite is the Mage-Knight noble. By giving the noble the Radiant gem of Hoeth, he gains a wizard level and can cast spells, and also wear armour, and swing a sword. And with the right spell, you can literally maul your opponents for cheap. I'm referring to bears anger from the lore of beasts. Being the default spell, and castable at 5+, you can use other mage characters to give space for the noble to get it through. And being a rest in play spell, if you get it off during the start of the battle, you might be lucky enough that your opponent forgets to dispel it before your noble gets into combat.

Another version of the noble is the Reaver Bow Noble, which basically makes the noble into a mobile turret, with just about the same killing prowess as a Repeater Bolt Throwers volley. This is based on the higher strength and ballistics skill of the noble with the Reaver bow.
This noble should opt to sit back at the deployment zone, since he'll boast an impressive range. Give him some good weaponry and he can make short work of enemy scouts and other war machine hunters. Basically, he can turn your RBT-guards from speedbump into quite the deadly assembly.

A third exotic combination is the Chicken Noble. Riding a Great Eagle and carrying the Star Lance, you're got a 20" strength 7 hit. It's a bit of silly build, but it has been used with great success. destroying enemy chariots before they're passed even half the map is always a laugh, and will most likely put a serious dent in your enemy's plan. As well as allowing you to focus your firepower elsewhere.

The fourth Build I'll mention is the Battle Standard Bearer. This is your only way of fielding the more expensive elven banners that can grant you a potentially devastating combat resolution, or even complete immunity to magic. Whether on foot or mounted, the BSB is mostly used to either keep one of your units in combat by means of removing psychological effects, such as fear, and by granting a reroll to failed break tests. Or to give a big punch in both static and active combat resolution from banner and blade in order to break an enemy unit.

Mind you, these builds are just examples, and it's up to you to create the best builds for your noble. Always consider the composition of your army when designing your noble.