That Time I Was Reincarnated as a Slime

Monster armies marching on a village or being intercepted in the wild is a fairly common set piece for fantasy and science fiction.  From the outside, it seems like it’s a simply obstacle to create and use.  Monsters rush forward, heroes stand their ground, slap in the action, and there you go.  Is it really that easy?  Only if you want it to be flat and boring, so let’s see some ways to spice this up.

1. It helps to come up with a reason for the monster army to be on the march.  This gives them more substance than being an obstacle.  It also allows for the heroes to know that they are either coming or in the area.  Some possibilities are a migration of the species, controlled by someone, and smart enough to attempt conquest.  It may require a little forethought, but it definitely brings more weight to the event.
2. Decide on how the heroes are going to encounter this army before it happens.  Think about all of the times you’ve read or seen a story where a monster army appears.  The heroes are either trying to get around them, defend a location, attack them first, or accidentally run into them.  Each situation requires a different way of thinking and tactics.  The tone differs as well with some having more tension and suspense than others.
3. The monsters don’t always need a visible leader, but it can help.  Even migrations tend to have at least one figure in the lead to act as a guide.  A more organized army would have leadership figures to keep the others in line.  Keep in mind that these are still monsters and not the more civilized races, so they’d go wild or run away at the slightest setback unless they are urged forward.  You can get around this with enchantments that force them to continue fighting, but that still requires a leader who may show up later.
4. If you’re having the heroes work with another group to face the monsters then you should really kill some characters.  Not the named ones, but the ‘fodder’ you’ve added to the scene.  This shows that the monster army is dangerous and there is a chance that they will win.  Otherwise, the audience will wonder why they should care or worry since even the nameless characters are safe.  The same goes for injuries, which can be done to anyone involved.
5. Consider the effect on the environment since monster armies tend to be massive.  They aren’t known for going around forests and villages.  Usually, these forces just steamroll over everything in their path.  Trees will be knocked down for weapons and anything edible will be devoured.  Think of the monster army as locusts.  Also, people should be able to find this path of destruction, which means they can run ahead.  This is how the heroes will know what is coming since it’s hard to hide a monster army.
6. Don’t be afraid to have the monster army swarm or surge even if they’re supposed to be organized.  This is a sudden, mad rush forward to push the enemies back or break through defenses.  I’ve heard some people say that this tactic wouldn’t be used by a formal, organized army.  I’m fairly certainly it would if they thought it would work or had to operate out of desperation.  It helps gain more ground and can terrify any enemies who are getting nervous.
7. To really create a great scene, try to touch on the senses aside from sight.  The monster army will have a fairly unique sound and smell.  Have the heroes react to these because they’ll be intense with so many creatures.  The air may have a certain taste once the fighting starts.  Think about other ways to drive set the right mood aside from saying the army is stretching into the distance.