I had an idea for a a part of the campaign I am planning, but there's one issue that I can find. My idea is to have a large scale combat encounter essentially simulating a battle such as vimy ridge. It would be taking place on a semi- advanced feudal world, so I was hoping to incorporate trench warfare and all those challenges into it. The issue is, how do I do this? I assume with a large scale battle that you couldn't feasibly control hundreds of individual units, so how would something like this work?

Well, the easiest way is figure out which group will win ahead of time and narrate it out, with changes whenever the PCs do something to make it better or worse.

The next easiest way is to basically give rough 'stats' to the two armies and have a roll-off to see which side wins. Give bonuses for training, equipment, positions, etc.

The more difficult but interesting way is to make these aforementioned stats for individual units or fronts in the battle. Have rolls randomize what happens when the 203rd Turd-Burglars Regiment charge the entrenched 2nd Furries Company on the eastern flank through no-man's land, the effects when artillery targets locations, etc.

After doing all that, make sure you factor in your players.

I don't know if you happen to have it, but the Tome of Blood in Black Crusade actually has a premade system for determining large-scale encounters. My group has been using it in that system often to simulate their assault on heavily populated worlds. I'm not sure how much FFG allows people to say in regards to posting their systems on the forums so I'll leave it at that for now.

Only war also has a good system for large scale encounters.

I have said this before, but there already is a system for running large scale battles with 40k itself. My group roleplayed the second half of the Siege of Vraks, incorporating our characters into 40k and running many of the scenarios in the Vraks books. Wasn't too difficult either.

I am also planning running the Apostasy Gambit and am going to add some 40k battles into the 3rd book to show just what is happening to Calixis during that book.

~ alemander

Only war also has a good system for large scale encounters.

As do Black Crusade, The Tome of Blood.