Orks have a very hard time against Tyranids, mainly due to the fact that boarding and ramming (two main tools of orks) are more problematic than usual.
Boarding isn't that unreliable, it is just hard to pull off, and it is never a sure thing. You may be stacking a +4 on your opponent, but they still could roll a 6 and you could roll a 1. Then again I usually don't board unless I have at least a +4 (most wont) Usually this works out if your opponent is crippled and has a BM in contact, and you of course are not.
Ork Kroozer with 10 hits vs. crippled IN cruiser with 4 hits, BV10vs6 +1, BM+1, Orks +1, Crippled +2. There's a solid +5.
I ram if my ships are doomed, maybe the explosion will help, in fact exploding is a decent tactic against the 'swarm' fleet. Although Tyranids are better at boarding, and since you're between rulesets it's hard to tell you what I would do. For the 'Revised' fleet I would
If he is using a ton of launch bays, he can't use bombers (the orks greatest weakness) so you can play little tricks to absorb ordnance, I call it the suicide ramships.
Basically a wave of ordnance has to interact with whatever it contacts. Ramships are cheap and quite maneuverable, so you fly one into large waves of ordnance, this will invariably kill the ramship, but your kroozers will be saved. Since he won't be doing much shooting, and Tyranid ordnance is widely invested in 'hit and run' vehicles, you will likely want to take mad meks, which help with fixing critical hits, and do what Tyranids hate most; shoot them.
Kill Kroozers (in BFG:R) are ridiculously cheap with long range weaponry. You can shoot the slow Tyranids long before they can hurt you. His ordnance kind of sucks, but you only really care about one critical hit (Prow weapon damaged).
Another option, if he isn't running tons of launch bays, is to max out on Onslaughts (or possibly Savages) and just outrun the slow Tyranids, forcing the 'hive mind imperative' to work in your favor. Note that your low leadership won't hurt you for All Ahead Full orders.
