I have a decent amount of experience playing Tyranids as it's my wife's army. Even if the lists are not always super optimized. I don't know what models you have access to and I know that's always an issue but these would be my suggestions.
What to drop:
On the HQ-
make him an El, not an O (you could take a TA instead if you wanted the 5 BS).
drop the SI
I could go either way on the flamer, in an ideal game I'd want to not really be in flamer range, though again, that is ideal and not always the case.
On the Fire Warriors
drop the photon grenades- you're losing assaults one way or another so you mind as well get it over with in one round so you can shoot at whatever wiped them out before they can assault again. These will only really help against the termagants anyway.
I like SDTs, but against this list they might not be super useful, or at least not worth the 160 points.
Other changes/swaps and what to add
3 XV8 "Fireknifes"
I would disband this unit and run two of them with the Shas'El as bodyguards, and take TAs on them to up their BS. With a lot of the points you are saving, I'd take two shield drones.
I would join these two together, take a team leader and two gun drones. To save points I might take the unit down to 5, as detailed below I think more missile pods might be useful, although this is a great anti infantry unit. Joining them together lets them be able to make better use of markerlights. ASAP, markerlight those hormagants and shoot them with these guys.
I would add two more to make the squad 8 in size and I might even give the team leader some drones so that your first couple of wounds are (hopefully) not taking away markerlights.
If you have the points (and models)...
I have not gone through and calculated points, but in general this is what I would suggest, four things you could use. In this order...
1.) More missile pods If you can fit a unit of two or (better) three XV8 "deathrains" in, do it. Give them some shield drones and have them shot missiles at things from the back all game long. I find that the missile pod is one of the best weapons we have against Tyranids. It really threatens all of their elites (except Zoanthropes) with armor 4, and the biovore. My wife takes biovores and they suck. They are practically designed to destroy Tau/Eldar. For me, they are my first priority. If she hides them out of line of sight, I deep strike the missile pod team, if not I markerlight them and shoot all of the missile pods I have at them until they are gone, don't let them last more than two turns or you'll be out of scoring units...especially with 3 of them, yikes. Missile pods are the best weapon to use against them. Once they biovores are gone the long range of the pods allows you to be able to shoot them at any monstrous creature on the board for the most part and the high strength means they are more likely to wound, even if they don't penetrate armor. With the the HQ squad and 3 Deathrains you could hope maybe by turn 2 (more likely turn 3) to be putting 12 missiles into a single monstrous creature a turn that are all either BS4 or BS3 twinlinked. The drones on these two units will help you have the 3 suits in each squad by turn 3, hopefully.
2.) Another Railhead- Again, AP 4, large blast plus burst cannon. I would really be doing what I can to have each Railhead take out a little hoard unit a turn, they should be able to manage it. While they are removing units of termies, your missile pods/plasma is shooting at the Tervigon.
3.) BSFs on all XV8s and hammerheads.
4.) Last, I would try to get the FW leadership up to 8 with a Shas'ui. And a BSF might not be a bad idea either. And if possible, I'd try to get one more squad of 9 fire warriors.
The biggest thing I can tell you is to get rid of those biovores ASAP. They can be counted on to remove a unit of kroot or FW a turn, so they need to go. Next I'd prioritize the hormagants and then the gargoyles. I don't see hormagants too much but against Tau I can imagine they will be devastating. If you can get rid of those three units, you can really outrange and out maneuver him (assuming the Tyrants are not flying). Try to stay out of range of his Hiveguard or have your stealth team deal with them, or a unit of outflanking Kroot. Keep your XV8s away from the Hivegaurd.
This is all my humble opinion of course. Good luck!