Level 140ish is where you should have all the points you need to fight her. You can, of course, attempt her earlier, but after level 140, more stats points don't help that much.

Waterfowl has three phases. Her first phase is difficult to dodge if you are up-close, but the second two are easier if you have the timing down.

Others will provide you with a "no damage" version of evasion, but I find that very difficult to pull off, so here is my "you'll survive" evasion technique.

You are going to press the dodge button 4 times. No more, no less. You need to time the first one pretty well, or you'll take a lot of damage, the second third and fourth are easier to time because they will probably co-incide with you're instinct to panic roll.

So, for the first dodge forward, you want to time a forward roll right when she starts coming down from her leap up.

The second dodge is a roll forward again... slightly delay your panic response, but not too much. The third dodge is another roll forward, go with your panic instinct. The fourth is a backwards roll, again co-inciding with your panic instinct.

You'll need a 51 poise for this technique to work, but you should survive so long as you have acceptable armour and 1200hp (so a full bar if you have only 35 vigor).

You might want to turn off the music and use audio cues to get the timing.