The Morningstar fight was balanced around being prepared for combat, equipping armour and bringing food, and using abilities from the action bar to endure the fight. To reinforce those mechanics, he hits hard to encourage you to use what you've learned, and better endure future combat.
But even with starting stats and no armour, you should be able to take 6-7 of Morningstar's attacks without taking action, so a 3-hit suggests something's crippling her.
It's difficult to say what might be causing the issue without seeing her action bar, skills and equipment, but a few suggestions that may help (which Turael should've covered en route to this point in the tutorial):
- If you use the threshold ability that blinks on your action bar during the fight, and eat food to keep your health topped up (the Eat Food ability you put on your action bar in the previous stage makes eating more convenient), you should be able to outlast Morningstar even though he hits hard for a new player.
- Equipping armour from a different corner of the combat triangle severely penalises your defence, so be sure she's not wearing eg leather armour while attacking with melee. IIRC Turael will let you know when you talk to him if you're wearing mis-matched armour.
- The bronze dagger and helm you've equipped up until this point should be sufficient equipment, but to make the fight easier, a set of armour will do wonders to absorb more of Morningstar's damage. There's an optional Burthorpe path that'll walk her through smithing a full armour set. (But it's basically the same steps as the bronze helm she should've already smithed on the critical path.) To kill him faster, smithing a bigger weapon than a dagger also wouldn't hurt (well, it'll hurt Morningstar).
- Turael should've set you up with an optimal Revolution basic rotation during the preceding combat tutorials, but just in case, have a chat to Turael and he should set your action bar to something suitable for Morningstar. It should include the threshold and Eat Food abilities at that state, in case something's happened to those.
- I'd suggest checking if her defence or constitution have been lowered by a temporary debuff, but Morningstar was balanced around level 1 stats, so I doubt that'd contribute.
- If you don't bring enough food to the fight, you should be given enough rabbit sandwiches when you enter the troll cave to eat your way through it. If in doubt, cook some more rabbit/fish. A backpack of food should be plenty to eat your way through the fight, even if you don't use armour and abilities.
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