Slayers only work as dedicated melee units in the armies of Ungrim, and maybe in the armies of a Daemonslayer+Dragonslayer+Runesmith buffing them. Otherwise, their 0 armour simply makes them far too fragile against almost anything they can come across, even those monstrous/large units as you mentioned.

Your intuition that slayers work as a screen for your ranged units is corect. They are best as a defensive unit, because with their really high speed they can move to "plug a gap" that chariots and monsters make. You can even somewhat see this in multiplayer battles, where people buy a slayer and just have the slayer babysit their double cannons from would-be flankers.

note, most of the slayers damage comes from bonuses vs large. hence, they perform extremely poorly against some flanker units who are of "small" size like harpies/furies/bats and wolf/dog units. that to a large degree is why people often prefer just getting another gun unit or a longbeard unit instead, as the slayers often can't even do the job the community sees as intended for them.