Hehehe... Tau are fast... but squishy!
Orks have T4... this is MASSIVE (mathematically speaking.) Get some Helmets for your Officers... Mesh Armor would help. BUT, since you play in heavy Cover, Lay Low will be YOUR friend. Think about it... your T4 Boyz could enjoy a nice 3+ Save.
LEARN the WAAAGH! as soon as possible... when you cal it, you get +1 M and +1A for EVERY Ork near the Nob. I have used it affecting my whole MOb... it wauzz purrty messy! Heheheh... you Charge 10" and most Boyz get 2 attacks with 1st hand and 1 with 2nd... and the Nob becomes a MONSTER! Since you are Assaulting, you got 1st Strike, so Eldars don't stand a chance...
One more thing... DO NOT BE AFRAID! What I mean is that hesitation will get you killed. If you send one or two Officers ahead... the enemy has NO CHOICE, but to shoot at them until the Orks are dead.
Instead send the whole Mob... GRETCHINS in front! Everyone out of LoS or Laying Low... the Gretchins just behind Cover. They are small, so they gain +1 Cover AND they become the Target Priority! The enemy has to shot at them... they are cheap and easy to replace... they are in your Mob to be abused, so abuse the Gretchin. Remember, every time you abuse a Grot, a Nob grows mo' teef!
Other VERY evil tactic... Send two Gretchins in front... behind Cover and Lay Low. If Cover is a 5+, the Grot has a 2+ Cover. Sure... the buggers do NOT count as Target Priority and since the save is so low, the enemy will ignore them. Then... WHACK!!!! Have them assault a critical shooter! They won't make a kill, but chances are they will survive and tie up that sharp shooter!
Funny tactic... THROW A GROT! It may not be very effective, but it IS fun! I threw a Grot 8" high... he deviated, but landed in the platform and survived. Suddenly the Eldar shooter had a Priority Target! The Grot did not killed the enemy... but he bought time for my 'Ard Boy to get there and make himself a Fairy Slushy!