I need some help with the reminder text on this card. Thanks for your help, as always.

Wow. My first hint would be to remove all of it. That's a lot of words!

Let's see. Oh right, it's the tidal / simple version of the werewolf ability. The known fix is to replace it with dual faced cards and trigger on spells being cast rather than having to track its state every turn. (Seriously, this was one of the things they were gonna have werewolves be, according to Maro's podcast. Before they thought 'better' of it.)

To keep the function as it is, but with fewer words... I dunno. "~ ETBs with a +4/+4 counter that has phasing"?

Just pure shortening: "At the end of your turn, if ~ has +1/+1 counters on it, remove them; otherwise put 4 +1/+1 counters on it"

Shifted to end of turn purely for the shortening; it changes when the rhythm fires, though. "At the beginning of your upkeep" is so horribly long.

Even simpler would be: \ ­: ~ gains +4/+4 UEOT, use this ability only once per turn.

Which gives you the "Bigass splashy green creature" effect without worrying overmuch about it. But that won't help if you want a LOT of this.

I think I'd go with Vitenka's "just pure shortening" option.

Rhythm 4 (At the beginning of your upkeep, if this creature has any +1/+1 counters on it, remove them. Otherwise put 4 +1/+1 counters on it.)

I think it's a simpler option to go off a variant of Alex's idea (At the beginning of your upkeep, if this creature has rhythm counters on it, remove them. Otherwise put a rhythm counter on it.) and cue off it like Monstrosity does.

As a side note, I think it's too convenient that this is a 6/6 on the first turn it can actually attack. I'd consider it entering wit a counter (which you might do as a separate ability, though), but maybe make the base version a 3/3.

~ ETBs with a rhythm counter on it.
Rhythm (At the beginning of your upkeep, if this creature has rhythm counters on it, remove them. Otherwise put a rhythm counter on it.)
~ gets +3/+3 if there's a rhythm counter on it.

It's easy to imagine Circeus' version resulting in something like this:

Flickering Shield
­
Artifact
Rhthym (At the beginning of your upkeep, if this permanent has rhythm counters on it, remove them. Otherwise put a rhythm counter on it.)
As long as ~ has a rhythm counter on it, whenever a source deals damage to you, prevent 2 of that damage.

That's a lot of design space to tap into, so if you wanted to drop the whole "Always gives a power toughness bonus", I'd support it.

On a side note, this mechanic has a bit of an uphill climb to perform. I don't think many people like the tidal effects from Fallen Empire, and while much of that malaise was directed towards power level, some of the problem was due to all the busy work. Sounds cool, plays rough. Not that I don't think it could be a fun idea, but I wouldn't make it a major mechanic.

Oh, and one last thing. Why not use time counters? I mean, really, why create a new counter when a counter that does the job already exists?

I used the text wording for echo "if this" so that it can go on other permanents and I used the format for the Myojin "as long as it has a divinity counter on it" and just substituted the rhythm counter.

I'm not using time counters because I want this mechanic to have its own identity and with removing all counters sometimes causing issues, I don't want it to interact with other cards in that manner.

I went back to using rhythm counters instead of a number of +1/+1 counters because I liked jmgariepy's suggestion that this ability could be made more versatile if it wasn't restricted to creatures.

This definitely shouldn't be a major mechanic for bookkeeping reasons, but I think there's some interesting design space here in the same vein as phasing but with a better power level and compatibility with modern rules.