Notes

If you can get to the corner near the water and cave wall you can stand and the Dagannoth Mother cannot reach you. Then you can turn on protect from missiles and take no damage. After that you can attack it with magic and range.

Tactics

Each colour she transforms into signifies a different weakness.

Orange means she's weak against melee
Green means Range/Special Attacks
Red means Fire Spells
Dark brown means Earth Spells
Blue means Water Spells
White means Air Spells

Examine

A horror from the ocean depths...