I have a party of 11 lv 8s for a one-shot I'm doing. They are soldiers in army A and they are fighting army B. How would you go about running the combat for two large armies? What creatures would you use? My idea so far is to just fill both armies with NPC soldiers. I'll only roll initiative for big bosses on the enemy army. I'm going to let my party hack and slash through the mob and take extra turns on normal soldiers as they make their way to big bosses of the army (Goblin boss, hobgoblin chief, golems, etc). I'm not looking for a tpk. but a decent challenge. Any tips or advice appreciated! Any recommended creatures would be great as well!

Good luck, no official rules for this.  You could try Matt Colvilles Strongholds and Followers, where they reduce mass combat down to a tactical card game and then have the players take place in battles that might reduce or eliminate a unit or maybe give your side an extra unit or buff them.

When running large swaths of NPCs, whether they be friends or foes, it's a good idea to simplify the health & damage rules. Don't think in hitpoints, but in the number of hits each creature can sustain. I recommend rounding health and average damage to the nearest 5, then dividing by 5 to get your health and damage for each type of creature.

For example, goblins have 7 hit points, and deal an average of 5 damage with their attacks. Rounding hitpoints to 5 and dividing by 5 gets you a creature that can deal 1 "damage" and has 1 "health." Or, if you're a Magic: The Gathering player, a goblin with 1 power and 1 toughness.

Additionally, instead of rolling and running each individual creature, group them. Stick NPCs into "squadrons" and roll for them as a group. So if you have 10 goblins in a squad, you treat them as a single entity with 10 health that deals 10 damage. Every time they take damage, reduce their health AND their damage output appropriately.

You can run large monsters individually, and you can let the players roll for damage, just be sure to round all damage to the nearest 5, and divide if the target is an NPC.

One more thing: LET THE PLAYERS ROLL FOR THE NPCS IN THE "GOOD" ARMY. Keep the players involved, even when dealing with things beyond the characters' control.

Or you can just play a round of Magic: The Gathering.

I would just focus on party encounters and have the clashing armies be the backdrop for the fights, because while various mass-combat mechanics exist, I don't think I've ever heard of a mass-combat ruleset that anyone was particularly satisfied with unless it was horribly complicated to the point of just becoming another game entirely. 

If you want to include an element of strategy in the fight (as in, if you don't want to just tie the outcome of the battle directly to the outcome of the player encounters), you could maybe have the players be "in command", and throughout the fight present challenges to the players like "the enemy have deployed a siege weapon to the third gate, will you move forces from the other gates to help protect it (and risk making the other gates more vulnerable), or will you let them fend for themselves and hope they've got enough manpower to take it down?" And then you could follow up the result of their decision with a coin-flip or a roll of your own with a DC determined by the orders the players give, you could do something like that if you wanna tack on something extra to connect the players to the larger battle. 

As for monsters you should use, I would say that's entirely dependent on who they're battling against. If it's an army of drow, then they would likely have waves of enslaved folk of various surface civilizations for canon fodder, giant spider riding cavalry, maybe even some demon support. If it's goblins, then you would probably have hoards of goblins directed by more organized hobgoblin troups, with bugbears and ogres as the vanguard. 

Essentially, every encounter the players face should contain some amount of the following: waves of little guys that don't present much of a threat outside their numbers (so players get to feel awesome and heroic mowing down bad guys left and right, and can really cut loose with their AOE attacks), more skilled commander units that are much closer to the players' typical CR rating, and big mini-boss monsters that will present a genuine challenge (maybe being above what they'd normally face if you really wanna push it) to heighten the tension after a few rounds of spending resources on smaller enemies. 

You should also feel free to mix these around so encounters aren't just like "ok here's 20 little guys, 5 commanders, 1 boss, rinse, repeat."

Funny timing...i'd suggest subscribing to M.T. Black's emails. Its basically just a weekly '10 things related to D&D email". The email he sent out today has a link to a homebrew option to solve what you're looking for (although probably not totally = to what you're looking for), here's the bullet:

"#4. A couple of weeks ago, I mentioned my quest for a solid D&D mass combat system. I recently came across this supplement, which treats armies like enormous monsters. It's an approach I like, though I haven't playtested it yet. "

 the problem with playing a group that is part of a giant army as the group's contributions aren't really measurable in the grand scheme of things (sure you can dream up examples to counter that statement)...you either play it out as a regular ol' combat scenario or you play things out at the army level.  You could always play out the individual scenario and then separately track the army battle...but then you're still left with the result of the individual party combat not really mattering in the grande scheme of things - so why bother.

The magic analogy is perfect thank you

I'm currently running a campaign very loosely based on 'The Romance of the Three Kingdoms' so war is raging all around.  We started off using the rules from unearthed arcana but, honestly, the party didn't get as much satisfaction as they thought they might.

We found it far more interesting to assign specific missions or discover them on the battlefield.  Fighting enemies in waves to prevent the opposing army flanking ours, holding a pass or a fort worked really well.  Preventing the flag falling into enemy hands after the flagbearer was killed proved to be really good and earned the party a lot of influence.  Protecting a general who had been knocked off his horse and getting him back behind our lines also worked well.