I feel that Witch of the Lake has too many homing projectiles.

[Bwixie]

As someone who've defeated her on NG and NG+, now attempting challenge runs, she just has too much spam that it's impossible to dodge perfectly. A tanky shield is practically required, especially since missing one dodge means you instantly die. I can handle other bosses' projectiles just fine.

Am I missing the gimmick? Do I not dodgeroll through? Darting through the air doesn't fair much better.

I think this is the true challenge of Oar/Iron Pot runs.

[Bankai9212]

Pretty sure the best strat is to stay near her so her projectiles fly past you.

[7FOUR]

There's no gimmick. You just have to position yourself properly, and hope she doesn't go too far to one side of the screen or use the wrong attacks in a row.

The most consistent tactic I've used to win without a shield is to position myself directly beneath her, and stop attacking as she begins to cast. If she does the projectile barrage, I go ahead and resume attacking since she can't hit me. If she casts dark arrows (red orb that fires projectiles after a delay), I follow directly underneath the orb, then dash back the opposite direction as it begins firing and until the attack finishes, then pray she doesn't get off a barrage before I get back under her. If she summons the dark swarm (black orbs), they can just be rolled through or jumped over without any issues. Her physical attack is easy enough to see coming and dodge (she'll usually try to get on the same level as you while lining up the attack), but mind that she will follow up her melee combos with dark swarm beginning at around half health.

One more little tip is to pay attention to her positioning. Depending on how high she is floating, you can predict what she is going to do sometimes: If she's close to the ground, she's going in for a melee attack. If she's floating just above your character's head, she'll do the barrage or a melee attack. Mid range is where it starts to become a slot machine, but assume that the higher up she goes, the more likely she'll do that annoying delayed orb attack. So despite what I said about keeping an eye out when she casts, if you are in a rush to burn her down, you can take a chance on keeping up the offense while she's closer to the ground.

If you get really desperate and your build permits, you can also try parrying the spells (Bloodbrow Scutum and Mirror Shield can do this). This works beautifully, but may take more practice than just figuring out the dash timing.

Aside from those tips, it's down to luck. The more NG+ cycles you go through, the more health she has, and the more chances she'll have to act inconveniently.

[yudrenujhren]

Seven Four told all right. I would add just that while dashing from dark arrows is better turn back to the witch and dart over remaining dark arrows. This move save you from possible following barrage.
P.S.: Most interesting boss in the game, I guess.