they're just about to get a new book so most of our advice is going to become pretty irrelevant if you're game isn't for a while yet.
in the meantime, we have two very big advantages over tyranids;
as it stands, you don't have enough transports to build a good list here, so i'd recommend maybe going for:
duke sliskus (proxy the archon here + deploy him with the warriors for better poison)
-raider with splinter racks + NS (lance)
x5 wyches (just keep them out of sight as best you can, they'll die hgorribly outside of a transport to anything with a gun.)
x3 reavers, blaster or caltrops
x5 scourges, x2 splinter cannons or blasters
depending on kit and options, you can make somewhere between 500-750pts here, but be aware, its not a good set-up by any means.
you need more transports. a venom for the incubi and a raider for the wyches would make your life much better here!!!
-essentially you're looking at a gunboat that can kill their big gribblies easily. use the lance on the transport first and if you have to get close, consider a blaster in the unit and on a generic archon (instead of the duke) in the unit as well. lances and splinter weapons go beautifully hand in hand against tyranids but against no other army lol.
the wyches are just there to sit on an objective and deal with and gants/gaunts that overrun your shooters.
the reavers can either take a blaster and shoot at big monsters or just turbo boost with caltrops to thin out their infantry.
the scourges can pump out a huge amount of firepower with x2 splinter cannons and avoid being charged too.