Now *this* is getting kinda dumb. Not the actual plan of attack they come up with, mind you, that's actually sensible use of Olivia's one-woman army capabilities; but rather the whole larger plan that sends them to the fortress in the first place.
Because at those numbers the garrison isn't going to even let the small vanguard force start laying siege in the first place, they'll just come out to fight and try to wipe it out before the main force comes up. Defeat in detail etc. IRL even "masking" detachements whose only job was to sit tight and keep garrisons bottled up behind their walls were for obvious reasons scaled to be too strong to risk serious sallies against. The whole *point* of fortresses is to act as force multipliers that enable the defenders to hold off superior forces for extended periods, after all; a superior force itself has little need to hide behind walls against an inferior one.
Moreover there's no point in sending a small advance force to test the fortifications in the first place - you do probing attacks as part of the actual siege proper to look for weak points and try to pressure the defenders into surrendering on terms. The role of such a vanguard "flying column" would be to take control of the surrounding countryside and cut communications which obviously demands sufficient mobility and strength to not get chased off by the garrison. Practically meaning cavalry, mostly of the cheap and light kind that can readily patrol large areas and evade heavier forces as necessary stiffened with enough proper heavy horse to discourage mounted sallies.
Leaving aside the ridiculously lopsided forces, the conduct of their distraction operation makes little sense. Height advantage means the defenders can simply shoot further even with the same grade of handheld ranged weaponry to speak nothing of mural artillery which handily outranges such even at level, and running back and forth like that would just put arrows in the soldiers' backs instead (not to mention tire them out and more likely than not do bad things to their morale). It's completely unnecessary too; all they'd have to do is start making visible preparations for an escalade which pretty much automatically compels the defenders to stand on alert, and does wonders to attract attention and most forces to the threatened section of the walls, no need to enter effective firing range in the first place.