There are excellent guides at www.gamefaqs.com.
But if you can't get there for some reason...
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Save your guns for the zombies.   The guardian is only vulnerable to a
magical  attack, and only at certain times.   These times are when she's
charging up and there's a ripply green shine going over her.   Put a red,
3-point magic attack on a shortcut button and a red, 3-point damage field on
another.   Other useful spells are shield  and recovery (health and sanity,
mantorok rune if you have it).   "Magick pool" is nice too (tier and
redgormor - spell 11 I think - you should have the runes for it even though
you don't have the spell yet).   Actually, you might set yourself up with a
7-point red shield (and then move around to rebuild your blue bar) before
opening the door.
At first, get quite close but not too close.   You want her to try to zap
you with that gripping lightning, not blast you with that vertical column
thing.   Try to sus out her pattern.   Hit her with the magic attach when
she's charging up.   You probably don't need to anticipate it if you use the
3-point attack but you probably will (need more time) for a higher attack.
A shield can help buy you time when anticipating.   A 3-point attack is
enough though.   After you damage her some, she'll summon zombies.   This is
where the damage field comes in handy.   Trick the zombies into walking into
it.   Or shoot them with your gun.   (Or hack them with your two-edged sword
if you have it.)   You don't want your precious next magical attack to lock
onto one of them.   In general, run around like a nutter to build up blue
and avoid the lightning.
After a while, she will have closed you in and you'll be running around her.
She will be trying to blast you with those vertical column things.   They're
easy to dodge.   Again, she'll be charging up sometimes.   Hit her with a
3-point magic attack when she's charging up.   Sooner or later you will have
to anticipate her charging up a little in order to get your attack done in
time.   A few hits like this and she's done.