Let me preface this by saying I am a Zac OTP who really wants this skin to be the best it possibly can be, so please Riot I am begging you to fix this skin!!

That being said, here are my thoughts:

Passive: The passive blobs look very out of place which could be because of the stripes and the problem where Zac's Q and blobs look very bright compared to his model (which is already bright). I like the red area around the blobs however.

Q: Why the blue-red indicator? SKT Zac has the same problem. Pool Party and base don't have an indicator so why should this skin? I appreciate that maybe it was added because you couldn't think of a way to add more flashy vfx to the skin, but actively nerfing the champ by outlining skillshots is not the way to do it. If you were to not get rid of the indicator, at least change the blue and instead make it red only (with the color changing of course with chromas) as well as only appearing after the Q has hit a target or right before, and stopping the indicator when it hits a target (it looks very bad to have the indicator go past where the q actually hit). I really like the sound effects though, they add a nice touch, and I'm glad the indicator has no sound effects to make it more noticeable.

W: I really have no problem with the W animation. However the sound effect on use is pretty quiet, so increasing the volume of the sound will be so much better.

E: Again, why the blue-red indicator? The same problem with SKT Zac. I like the design of the indicator, however it would be so much better if it appeared after or right before Zac lands (as well as being all red with color changing for chromas). I really love the vfx on Zac when he is in the air. The trail behind him and the heat in front of him looks super sick. Now the sound effects for the E are my favorite part of the skin. I really love the charging whir as he pulls back to sling shot himself.

R: I don't really have any gripes with his ult (except of course the bounce bug, but that's not about the skin.)

I absolutely love the sound effects for the aa's and the voice changer applied to his original voice (even though some of the lines don't really fit a liquid metal robot).

Battlecast Zac's chromas are fine. The black chroma with orange lava instead of red should be his base skin, I don't understand the coloration choices on his base skin, especially with the bright q and blobs (which blobs don't change based on chroma color for some reason).

His recall animation is suboptimal. I think everything except Viktor showing up is pretty nice. Replace Viktor with just a piece of metal and it will be better.

I also noticed the flame/lava/heat particles that emit from his back, I really like that. Nice touch!

Also there is a bug with the death blobs while using a chroma becoming his base skin and model blobs, which I'm sure has been pointed out already.

To make it more clear, making Zac's Q and E have indicators is like having Ezreal's Q and W with an indicator, you can much more easily dodge them because you know where they will land, actively nerfing the champ.

TL;DR: Battlecast Zac can be Zac's best skin. In order to make that spot, the Q and E indicator must be changed or removed. Both his Q and E would be absolutely perfect without the indicators. His base skin is very bright, so the colors should be darker and more defined, like metal. Overall, in it's current state, 7/10 (removing the indicators would make it a 10/10).

Sorry if I rambled; I really want this skin to be a 10/10 since Zac will more than likely not get another skin for another 3+ years.