We have a LOT to cover on this one, so let's get to it. To see the consistency of this strategy, you should watch the whole video.
For viewing the strategy alone, along with Apex Waterwing Heart Drops, you can skip all the way to here 18:13 - 19:55 and 20:09 to show you this is done on Ultra Hard NG+, along with the used Equipment.
The start of the video to 9:30 shows that even reloading the game might only give bad results.
Unfortunately, the Hearts do NOT have a 100 % Drop rate.
From 10:30 the run finally gets frequent Heart Drops.
First of all, spawn at the Campfire at the grave site just outside of Fleet's End.
We need to be able to take down each Apex Waterwing as fast as possible, without alerting other machines for as long as possible.
With Low Health (LH) Ranged and Defence, Aloy will hit really hard and be tough to go down.
The Tenakth Vanquisher (Skirminisher as alternative) is the best option for +4 LH Ranged and Defence.
Otherwise, you'll need LH Weaves and/or Food as alternatives with other Outfits.
Be sure to prepare a Weapon Quick Swap with the Square button on the selected Advanced Precision Arrows of your best Sharpshot Bow.
A Boltblaster will alert all Machines and the last thing you want is a group of Waterwings charging at you... Trust me.
Once in the Critical Health state, fly towards the beach just a bit North of Fleet's End.
At 0:30 the Waterwing we'll be targeting, is a regular spawn.
You can do two things to make the Apex version spawn. First, fly away far enough for the machines to suddenly disappear, like at 0:47 . 0:58 If the regular keeps spawning twice (or more) you can just reload the game and try again.
The second option is to Fast Travel away, save and reload, then travel back.
1:49 Once the Apex version spawns, make sure you've equipped an Outfit with +2 Low Profile and Quiet Movement perks for a silent approach to the Tallgrass here 1:56 .
Once you're in the grass, keep crouching and equip the LH buffing Outfit, in this case the Vanquisher.
2:18 Wait for the Waterwing on the left to fly off and the Burrower to go more to the right.
2:28 Pull out The Tie That Binds (or the Elite Ropecaster), preferably fully upgraded for a single rope Bind down.
Fully Overdraw the Ropecaster and hit the Waterwing.
Once it is tied down, Freeze it with the Sun Scourge (Seeker Hunter Bow as alternative).
I do not prep a Triple Notch for the Frost Arrows to show you that there is plenty of time before the Burrower starts noticing you.
Of course, DO prep a Triple notch yourself before doing this for a faster Brittle state to happen.
Quick swap between each shot, using two L1 taps, for instant reloads of your Frost arrows!
Once the Apex Waterwing is Brittle, pull out your best Sharpshot Bow with as many (Elite/) Crit Chance Coils as possible.
If you don't have the Iriv's Downfall, the new Gravesinger's Lament, Delta SS Bow and Regalla's Wrath are solid alternatives.
No more than three Crit Chance 15 % Coils? Then use either the Ranged Master or Critical Boost Valor Surge, for improved dmg and/or Crit Chance.
2:57 With the new Weapon Skill in the Trapper Branch, you can tie down a Machine again for a bit more time before the machine can move again.
~ 10K dmg should be enough to kill an Apex Waterwing.
3:12 If you are noticed, even if there's no machine alert, either shoot down the approaching Burrower in the Eye or the Soundshell.
Or quickly loot the Apex Waterwing and throw a Smoke Bomb.
Then fly away and return to redo the loop from the beginning.
If you need a Valor Surge for each loop, you'll have to refill your Valor Bar again.
Silent Striking (+4 needed for one-shots on most Burrower versions.) a couple of Burrowers or Scrappers with Valor Surge Master enhancing Weaves and/or Food will easily return Valor.
3:15 Unfortunately, the loot is terrible. NO Apex Heart...
4:36 And the regular Waterwing appears again. Reload game it is.
That concludes the strategy. The rest of the video is to show you how to get more consistent Apex Waterwing spawns, along with Apex Waterwing Heart drops.
5:09 - 9:30 STILL no Heart... We'll have to Fast Travel to a Shelter, Restock ammo, save, reload the game and return. This will allow for the game to chance the Loot Drop Box of the machines. Aloy dying should grant the same effect, but would you reall- Wait, nvm.
11:19 Here's the first example that does NOT go as planned.
11:47 The Strat begins. 12:20 Machine Alert.
12:23 Loot, Smoke Bomb and RUN!
The next run goes even worse, but LH Defence saves us twice from a one-shot and one more save with lucky Rng.
The following loops, most notably 18:13 and onwards, go as planned. Plenty of Crits to kill the Apex Waterwings quickly and they drop Hearts plenty of times.
If you have any questions or suggestions for a better Waterwing strategy, feel free to comment! :)
Good hunting!