My party has recently been sent to the front lines by an opposing inquisitor trying to get them killed and I was wondering if there was a better way to simulate a large battle other than giving hundreds of friendlies and enemies initiative and then rolling for every attack...

I'm thinking making a few GM rolls for stray shots and artillery and just making sure they survive a certain number of rounds (I don't intend them to fight a whole war Merely this specific attack).

You said you wanted to simulate a whole war, but then at the end you said only one attack. So I do not understand which is your question.

A war is made up of small battles.

They have been sent to planet where a global war is taking place - they will only have to survive one attack before I narrate them away.

I just want to have a suitable mechanic for this particular firefight.

Just have them fight 10 enemies charging towards their position on the front line, or maybe a few enemies and a vehicle.

Creating a good narrative of a battle raging on around them can help contribute to that feeling of being in the midst of large scale conflict. Describing artillery strikes, armored column assaults and the like, even if the players aren't directly involved, can help simulate it. Throwing in a few friendly NPC's such as Imperial guardsmen into the actual fight the PC's are involved in can make it feel more battle-like (assuming of course you're willing to commit some more time to the combat). And building on what Utherix said, maybe throwing something insurmountable at them after the main fight concludes and getting them to fight hopelessly against it for a round or two before before narrating them away could give the impression of a greater battle raging on around them.

Try playing a game of actual 40k to simulate the battle

....It could work.

40k? What do you think ... We're MADE of money?? lol
Yeah- I guess the narrative description will definitely help.

I'll give them some explosions to dodge and maybe throw some soil at them.

Otherwise you could set up a mock battlefield with a program like rolld20, or even Paint if you want (rolld20 is probably better for this though). Count enemy figures as units, companies or whatever, and same for the good guys. Run the battle like a narration, keeping track of the stuff on the battle, and doing mass rolls-- that is, roll maybe one test for an entire platoon at once, sort of deal.

Then use these results to guide your Acolyte's through the battle.

The campaign I am in recently journeyed to Vraks and we made up 40k versions of our characters via the 40k rules and ran many of the scenarios in the back of the Forge World Vraks books. It worked well.

~ alemander

I use to run a Rogue Trader game in which Mass Combat was fairly recurrent. They have a couple of different rule sets for this in the Core. If I remember correctly, squads were separated into dozens of combatants who are all using the same weapons. I think you rolled the normal damage roll for each attack+the number of guys and gals you had left and that was the amount of casualties your target would sustain. It was simple, fast and efficient with lots of room for gory details but very little for actual strategy.

For what you are intending you don't need to worry about how the two sides involved get on as it isn't central to the PCs story and more importantly I get the impression the PCs aren't intended to influence the result.

Therefore narrate the battle as you see fit. Throw in a few crazy moments (a transport lander gets blown up in the air and it starts raining chimera APCs for instance). Roll dice frantically and then look relieved as you describe the near misses of the PCs. Make things like artillery shell's snipers and aircraft straffing runs environmental hazards, and then have the PCs face their enemies. Run the battle hard and fast. And then narrate the PCs out. In other words you aren't looking to simulate a battle you are looking to railroad the PCs in am interesting manner. I don't mean this in a bad way. Remember tropes are not bad.