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I played an Omega level recon game to start of my latest 40k tourney against the necrons, and this time I decided to field an orks army. The problem I ran into was an Immortals unit with a Necron Lord carrying a rez orb and a veil of darkness. How the hell am I supposed to stop a unit like that? That unit shot the hell out of my mobs, and then, right before I could get into CC he would use the veil to run away. And on the last turn, he used the veil to get into my deployment zone (bonus VPs). How are orcs supposed to stop a unit like this? Even a wartrukk moving all out speed couldn't do anything about them. The Immortals outrange most of my men, and I can't get into CC, plus I need to kill some of them for a phase out, so there is no way around them. My army is two killa kans (rokkits) two warbuggies(rokkits), two trukk boys mobs (nob, rokkit, burna boy), and two large footslogger mobs of slugga boys. I decimated his army all game, but that one unit won the game for him (and his destroyers). I would love to know what to do against these guys in the future. His army was a lord with the immortals, two units of warriors, and a unit of 3 destroyers.
It seems to me you may be using the wartrukks incorrectly. If you moved up both of your trukks about 12" or so first turn into a safe, concealed area that you could charge from next turn, with the buggies shielding one/both of them or distracting the enemy, you should be able to get one if not both of the trukk mobs into combat with the immortals. Assuming you gear them up properly, they should be able to take out the immortals, if not tie them up for most of the game and allow time for the rest of your horde to move up the front.
Another suggestion would be to use a looted leman russ of basilisk. Either way (res orb or no res orb) they are going to get their same WWB roll, but that doesn't make it useless. You will still be able to take out 3-4 immortals a turn by successfully landing a pie plate on his squad.
Even a relatively dim-witted necron player can be a difficult opponent for even veteran players, and necron player with some cunning can be devestating. That is all I can think of in the ways of helping you, i know they aren't great but it may work with a little bit of your own touch. I'll post again if I can think of anything you can expoit.
On second thought, can you post what he uses in his army, so we can perhaps help you out further?
"...a swarm of mechanised locusts sweeping over the land, stripping it bare of resources, bringing death and destruction to anything that stands in its path. Emperor preserve us against the predations of these so-called Orkish cults of speed!" - Cardinal Nomura at the Conclave of Hessen
The Veil can teleport out of combat which also sucks. Trukk boys are fantastic against the unit, also you can always ignore that unit and just commit your boys to getting stuck in with the rest of his army. Once your in combat, you are completely safe from a veil unit as they can't teleport and charge. I would just commit yourself to mobbing his other units and force a phase out.
40K-Beakies(9-14-4),Guard(4-7-2),Orks(34-12-11). FANTASY-Dwarves(15-6-7),Beasts (14-14-1), Skaven (17-17-10) DoC (6-1-2). CYGNAR (28-15-1)