Hostages? Sharing the Shake it Off teamwork feat? A couple squads of archers specifically tasked with readying to shoot casters?
If you want something to boost a large number of low level minions a skald is actually better than a battle herald. Ragesong boosts both STR and CON so gives your minions more HP. The skald can also grant any and all rages powers they have. You can take the feat extra rage power to gain more of them. Skalds get access to the 3rd level spell Flexible Fury that allows you to swap out rage powers, giving you the ability to tailor your rage powers to the situation.
Skald also continues to advance the spell casting and other class features of the skald. This gives you access to more powerful spells. Spell Kenning gives you access to spells from the Bard, Cleric or Sorcerer/wizard list. Clerics get a lot defensive spells including communal versions of some of those spells. An 8th level skald could give 8 minions 20 points of fire resistance by using a 3rd level spell to cast communal resist energy.
If you want something to boost a large number of low level minions a skald is actually better than a battle herald. Ragesong boosts both STR and CON so gives your minions more HP. The skald can also grant any and all rages powers they have. You can take the feat extra rage power to gain more of them. Skalds get access to the 3rd level spell Flexible Fury that allows you to swap out rage powers, giving you the ability to tailor your rage powers to the situation.
Skald also continues to advance the spell casting and other class features of the skald. This gives you access to more powerful spells. Spell Kenning gives you access to spells from the Bard, Cleric or Sorcerer/wizard list. Clerics get a lot defensive spells including communal versions of some of those spells. An 8th level skald could give 8 minions 20 points of fire resistance by using a 3rd level spell to cast communal resist energy.
You cannot share rage powers not gained through the extra rage power feat. It is GM discression wheter or not you can share the powers gained via the flexible fury spell.
"If the skald has rage powers from another source, he (but not his allies) can use those rage powers during an inspired rage."
stow them in a portable hole with air, food, and water?
I think what another source refers to a class other than skald. So, rage powers gained from levels of barbarian would not work, but for a single classed skald they should be able to use grant any power they have as long as it does not require the creature to spend an action or rounds of rage to activate it.
It is on the list but is a 3rd level spell.
Ultimately, large groups of individually weak creatures are not likely to be a significant threat. They will probably end up being "cannon fodder" for AoE attacks or just a way of delaying the PCs for a round or two from ganging up on the BBEG.
You should probably reduce the number of low-level minions and add some mid-level "lieutenants." A spell warrior skald "lieutenant" for the battle herald that concentrates on counterspelling can be extremely frustrating for the PCs. Add another "lieutenant" as a buffer/healer (many options) or buffer/controller (a master summoner to bring in even more minions, for instance) and the encounter will be less of a speed bump.
This type of an encounter does not have to be just about giving the party a major challenge. This type of encounter is great for bleeding of resources before the big encounter. Assuming the party does not get a chance to rest and regain spells every spell they use is one less they have available for the tougher encounter later.
An 8th level Skald with the Linnorm Death Curse, Tarn would be a particularly nasty encounter. Have the Skald take the spell Clarion Call and his ragesong can affect an entire army. Each minion the characters kill mean they have to make a saving throw or lose the ability to be healed or heal naturally. Since a 1 always fails on a save, if the party kills enough minions they will be affected by the curse. The wizard is actually more at risk from this than a martial.
Give the Skald Major Image to allow him to create illusionary troops and the party will be forced to burn even more resources. Mix the illusionary troop with the real troops to give the party a nasty surprise and confuse the numbers.
Invisibility Sphere could allow a small group of minions to get very close to the party without them realizing it. The skald cast it on the leader of the squad and uses Clarion Call to affect the squad and the other troops. The Wizard might have a hard time getting that fireball off when he is being attacked by a bunch of berserkers.