This monster is exactly what it says on the tin - a massive zombie army. A huge army of undead is a classic fantasy trope, but it's just too unwieldy to have something like 100 mooks on the battlefield. Instead, this writeup treats them as one big monster.
The zombie army works just like you would expect it. It's a giant damage sink, with roughly seven times the hit points of what high-champion / low epic PCs would normally fight. The best tactic is to throw as much damage at it as fast as possible, to decimate the zombies and bring down the number of attacks. Fancy effects won't do much, as anything that isn't damage just slows it down a bit (-2 to attacks).
|Size/Strength Huge, quintuple; Level 8
Role Spoiler (Lunk); Type Undead
Foul stench: At the start of its turn, the zombie army deals 5 poison damage to each engaged or grabbed enemy.
They're everywhere!: If you're nearby the zombie army, it can attack you as if you were engaged to it.
Twohundred arms and thirty-thousand teeth: The zombie army makes one attack for each 100 hit points it has (minimum one). It can attack each enemy only once.
Trample them down and drag them in +13 vs. PD—25 damage
Natural odd hit: the target is hampered until the start of its next turn.
Natural even hit: the target is grabbed.
[against grabbed targets] Sink your teeth in +17 (includes +4 grab bonus) vs. PD—30 damage, or 50 against staggered targets.
Never run out of targets: You can choose the zombie army as two (or three) targets of an attack. Make one attack and damage roll, and double (or triple) the damage.
That won't stop them: Any non-damage effect of an enemy attack is changed into dispersed (-2 to attack rolls) with duration until the end of its next turn. Multiple dispersed effects don't stack.
Separated from the pack: If a zombie or two get separated from the army, treat them as level 1 Zombie mooks and remove their hit points from the army's total.