I play a shooty dex ork list and I have problems with nids too, they are just real fast and can be really nasty shooty, especially with those barbed stranglers, ouch!!!!
I only use 1 trukkah squad and 1 burna boy squad for counter assault and I find that the trukkah's can do a number on whatever they charge but don;t have the numbers to hold up against the waves the nids can throw at me.
My burna boyz ussually hang near my Battle wagon so the mek can provide KFF, tool support and then when the time is ripe load up for some flamin drive bys. I have learned that it's best to keep the burnas in the wagun, DON"T GET OUT!!! The nid player will get very annoyed with all the burnin and focus a lot o firepower on it.
I try and use my tankbusta's and 2x rokkit totin kommando's to tag team a carnifex in the first turn (the ones my friend runs are gun fex's with minimal upgrades) this either succeeds in takin one down in fair short order or else ties up the beast as he either manuevers out of range or hunkers down in cover and returns fire.
The stealers, especially the ones with extended carapace are murder and very scary!
My best advice is to stay concentrated and avoid splitting your army over terrain, the nids can move so quick they can deny a flank and over whelm you with hordes of bugs. That and forget the big ones, the little opnes will bog you down fast, kill the blighters as fast as possible!!!
Oh yah a looted Russ or Basilisk draws a lot of attention!
My 20 shoota boys actually killed a winged HT in hth once! I totally lost the game but the fact that these guys kil a HT and 16 gaunts made me WAAAAGH!!!!!