Staying neutral during a siege

[bluntpencil]

A army of 100 Human Archers fought as a single encounter with no class features or feats, just proficiency:
STR 12, DEX 14, CON 13, INT 11, WIS 10, CHA 9
Longbow: To hit: +4, 1d8+2 [7] Piercing

Vs.

a level 8 party with a "classic" loadout of 1 cleric [AC16, HP60], 1 wizard [AC13, HP45], 1 rogue [AC16, HP60], 1 fighter [AC16/18, HP75]

If the archers split evenly, then about 10 hits are registered on each PC, resulting in 20+10d8 [70] damage to them dropping all outright except the fighter, who will certainly die in the next salvo.
If the archers split their fire a little more tactically and count on averages, moving shots from the squishy wizard to the fighter, they can drop everybody in a single round.
If the archers play it safe, they can just outright execute the cleric and the wizard, preventing any chance of blast effects reducing their number beyond the 3 that can be killed by the fighter and rogue or healing un-doing any of their work, and then execute the remaining two on their turn.

Even if the PC's go first, and open with fireballs or other large blast effects, they don't deplete the NPC archers anywhere near fast enough to avoid just being shot down in about 3 turns with maybe a dozen or two dozen people killed.

This isn't an unexpected result, since even with these like CR1/8 or 1/4 guys the encounter CR is like 17-25, which is way too high for a level 8 party even considering that CR is questionably accurate on the best of days.

The only real survival course of action for the party is to run. There aren't really that many tricks they can pull, but they can at least D-Door into the long range bracket, hope that the disadvantage causes enough misses that they don't get shot down, and then run from there.

Um, are we just assuming that adventuring party isn't casting spells like Invisibility, and Fly, to stay at maximum range for the archers? At 8th level, they'll fly in, float above them invisible, drop a Fireball and then move to where they're shooting back at -10, or they could use a Heightened Invisibility to get a whole minute of dropping hostile spells on them before retreating out of reach. That's before you even start summoning Swarms of low level bugs that are effectively immune to Piercing damage.

A level 8 Wizard, given some time, should be able to deal with them all solo. Maybe not in a single day, but he'll manage eventually, without taking damage. Even if time is limited, he'll do ridiculous damage, probably enough to sap their morale and scatter them. The rest of the party can stay home, or juice up on Potions of Flight and Invisibility if they feel they have to.

All this is before you simply fly above them dropping rocks or alchemist's fire on them.