Assumptions:
The IG have the turn.
There is adequate cover for all ground units.
Only the units listed are the units on th table (i.e. No drones for the Tau nor Commisars for the IG etc.)
The Fire warriors are 6-man strong, the conscript are 18.
You will need some of the emperors luck to have a fighting chance
Step 1: The conscripts should run -hugging cover- towards the enemy objective. If they stay in cover and have a good spread, nothing the tau has will inflict any serious casualities on them and you are free to taunt him for being a silly goose who wastes shots on conscripts. Even if both crisis suits squad land and fire all they have at the poor sods only ten conscrips will die (rough statistics) and wasting either the hammerhead or the firewarriors on his is -again- silly. Railguns will do nothing and the large blast has too high a chance of scatter (along with a 4+ cover save) to do any real damage. Hence you get not only a good blob to swamp and tarpit any batle-suits and/or bradsides, but you can also hunker on the enemy objective if you can reach it.
Step 2: The leman russ and the lascanon-squad both fire at the broadsides, there is roughly a 75% chance that at least one of them will go down since they lack shield drones. (Two shots with 4+ to hit, 2+ to wound, insta-kill)
Step 3: Ignore the fire warriors unless they are within rapid fire range, if so kill them with mass flash lights from your own rapid firing units. If they are in the devilfish, which i highly probable (and hence have a chance of closing in on you next turn for a rapid fire) use the autocannons to hopefully score a stun or better. Statisticaly (again, roughly) you should have a 50% chance of scoring a glancing -or better- hit from your four shots. If this fails don't worry. With six rapid firing warriors you will still only losse about 3 gudsmen, which no biggie.
Step 4: The Valkyrie needs to have as high a chance of survival as possible and should probably turbo boost, with luck the enemy will only have a single broadside and the hammerhead left, and waaay more targets than guns so hopefully it will be safe until next turn. Deploy the Storm troopers either on near the broadsides (in cover) to draw fire or keep them on board if you feel certain that the Valkyrie will survive. The reason for this is that this is your only reliable counter to the crisis suits. The flyer and the units within will make an absolute mess since you have AP3 hot shots as well as the two hellstrike missles. Idealy you should boost to a position far from your own objective but near the enemies, as any Crisis suits landing to take out the Valkyrie will then be too far away to thraeten your own objective in the remaining span of the game, while your stormtoopers will shoot up the crisis suits (or assault) and then go in to contests the enemy stronghold. If you have too few troopers to score a good two dead suits (Statistically you'd need eight Storm troopers) you can run up to the objective an go to ground. With a 3+ save the tau will either have to CC you or waste a lot of fire.
During the tau turn I'll assume that the Russ as well as several guardsmen die, probably at least one HV-Weapon squad. If the gunsihp goes down = oh dear. Anything else is prety much expendable as long as you're holding your own objecve.
In turn five you have to drop the Storm troopers and fire them plus everything from the Valk at one of the crisis suits. The conscripts (who tend to get ignored and should still be resonably near full strength) should keep running towards the broadsides, the objective, or -if the Tau player was really unlucky/stupid, try to assault his crisis suits. The Guardmen and their weapons should focus on the devilfish this turn (if it still lives) as a single broadside will do nothing now that the only target is a Valk without missiles and it's far more important to deny the enemy mobility. If you've focusd fire right (and had a good ammount of luck) the only thing that can reach your entrenched objective should be a single unit of crisis suits. If your concripts or storm troopers manage to deny the enemy their objective (just keep hugging terrain and the hammerhead will score mabe 2 kills per shot -essentially nothing-). You've more or less won at this point. At worst you'll end up with both sides denying their enemies the objectives and you'll have a nice draw at least.
This is of course based on quite a bit of luck, but times were pretty dire considerng what the IG had left so you may as well go for broke. Also, this math is done off of the top of my head, so a few mistakes may have snuck in