Akatsuki and Blitztank can parry moves with their DP

If you parry from non-point-blank range or parry an assist
you can negate it without activating the follow up attack

After a successful parry, you're invincible for a while

This lets you do things like
Avoid air unblockables
Avoid assist oki mixups
And control space

It also lets you avoid active switch mixups

Most assists don't last very long
The parry timing is the same for all of these

Other assists need some more finesse

Jin 6P needs two parries up close
and one once it starts moving
You can do a third parry for some extra invincibility at the end
Be careful not to parry too early

Rachel 5P has 6 hits
But it's much easier to parry one hit and let the invincibility deal with the rest
Like Jin 6P you can also parry the last hit for more invincibility
Just make sure you don't miss it
Also make sure you don't attack before the assist stops
However, you can use 214BC to trade with the assist on purpose to make it + on block

You need to parry this twice up close
But just once at a distance
Keep in mind that the later your parry hits the less invincibility you'll have
If this happens, just parry again

Makoto 5P is pretty standard for multi hit assists
but you can hit her right before the uppercut
This can be good for getting out of cross combo pressure or catching your opponent off guard

Izayoi 6P activates after a parry so you'll need to parry it more than once to get out

Platinum's 4P is random but you can parry all three versions if you time the parry right after the startup recovers
Her 6P is also random and harder to parry. You'll need to try and react to each one
The rod will either be yellow for the bombs, blue for the missile, or platinum will do a windup for the ring

Es's 4P is big and slow. If you see it you can basically air dash in for free

Susanoo's 6P is basically a laser so react to the startup

Normally, you can't move while you parry which can make fireballs like this hard to hit
However, if you parry while jumping or airdashing you'll move forward which can make it easier to time

After parrying persona attacks, it's a good idea to hit them with 4A or 5A and jump cancel to keep yourself safe and do some damage to the
Keep in mind that even if you parry a persona, they'll still block fireballs

Unlike normal assists, persona assists won't have added hitstop when parried
This means, even if the assist does multiple hits, you won't be able to parry multiple times

if you parry Teddie's 5P all 4 times, you can hit him from behind with 4A and jump cancel to stay safe

Don't forget that Teddie's 6P hits once as it flies and once when it explodes

Even barrel and buckets aren't as much of a threat as usual

The startup of Naoto's 6P is longer than it looks so try not to parry too early

As soon as you see Adachi cover his head, hit the parry button. It tracks anywhere on screen so you need to be ready to parry at all times.

Elizabeth's 6P is slow like Naoto's so don't parry too early. However you can parry late and still go through it.
If you parry late when she's in awakening, you'll need to parry it twice

You can hit Wald out of 6P with 4A if you wait a bit after the parry

Be careful. If you try to react to Carmine's 4P and miss the first hit, the second will hit you

Carmine 6P needs two parries unless you time it perfectly

It's really easy to hit Gord after the first hit of 5P
You can also do two parries to mix it up

Orie's 4P will always come out so parry it whenever you see it startup
It's also a good idea to parry it to avoid air unblockables

Unlike other multihitting assists, you can only parry Merkava's 5P once.

Usually you only need to parry a projectile assist once
Most projectile assists are either too fast to parry every hit or are just one hit, but Vatista's 6P is slow enough to parry every hit and completely negate it
Usually this isn't too useful but it can be good for protecting your assist

Normally you don't want to hit Vat 5P in the middle
But Blitztank can armor through it

Try not to get mixed up between Yang 5P and Yang 6P
Parrying Yang's 6P too early will get you killed

If you parry Blake's 6P, it'll immediately retract

Yumi's 6P is also slow enough to parry every hit

You can't hit Yumi in the middle of 5P so make sure to parry the last hit
Unless you're Blitztank

All of these examples are in the vacuum of training mode and reacting to assists in a real match is much harder when you have to deal with the point character too.
Parrying an assist isn't a guarantee that you'll have the advantage.
It's like activating resonance; you're invincible for a while, and you might avoid something, but the point character can set up a mixup while the invincibility wears off.
That being said, varying up the timing of your attacks out of parry, which attack you do out of parry, how many times you parry, and even whether you attack or not can really keep your opponent guessing
Thanks for reading