How can I be effective as a low level human magic-user in a boss battle against a more powerful magic-user?

First of all I’m a human magic-user level 2:

Except for the spell sleep and charm person and some other spells that were useful for X situation out of combat, (Enlarge person, Mount etc.)

I could not do much things, there’s no shops around for now because we’re out of town.

The only thing I have is a dagger and a sling (-5 to hit)

We are against flying birds humanoids and I killed 2 of them via the sleep spell (I used enlarge on our fighter earlier) because they were flying and they fell to their death, an NPC had a flying magic wings item and I took it to do so, luckily I was not as high in the air as the other enemies because my DM made the enemy magic-user (that we weren’t aware of) sleep on me and I lost 8 hit points (thanks to my cat familiar and constitution (and luck on HP roll) I have 15 HP), the enemy turned invisible and did some other tricks before turning invisible again, I (and my cat) could pinpoint where he is via superhearing, but I feel powerless since he went back in his tower and our party nearly died and we’re escorting lots of NPC and the enemy has 2 children that he kidnapped in his tower, most of the party is Chaotic Neutral but I’m Neutral good and our NPC healer is Loyal-Good, I think I have no choice but to abandon the poor children to their death (we’re not even sure that they are still alive).

There was still one bird left (15 to hit is required) so with my Sling 19-20 in my case (16 Dex)

The magic-user enemy is at least level 5…

Sleep is very powerful and as a human I guess the only thing that can protect me is my party and some future magic items/spells… unless I could learn some useful spells I’m not aware of…

How many spells does the Magic-User have in their spellbook in S&W Core?

In Swords & Wizardry Complete, the rulebook states that

[…] a beginning Magic-User’s spellbook contains as many of the eight basic first level spells as the neophyte character can know [based on their Intelligence score.]

Swords & Wizardry Core seems a bit more ambiguous:

[…] a Magic-User owns a book of spells, which does not necessarily include all the spells on the standard lists.

How do you determine the number of spells a Magic-User has in their spellbook in S&W Core?