Grimnir advice (help)

[Hase]

Hello, Im actually playing with grimnir and I need some advice.

Except his forward + U skill, how can he punish someone who spam his dash + L ?

The character is to slow to punish this action, light attack frame 6, down light frame 6.
Every action is too slow punish this spam dash attack + L (or M), so he keep pushing me and once I'm at the wall, It's just impossible to escape except his Except his forward + U skil or use a BP.

Some character have his own escape like Zeta with his skill but even if in some case I have the possibility to do any action after his dash attack, I can't do anything with a lot character because the range any normal attack (L or M) is to short to hit him (guard or not).

[Fish]

You can try out-zoning the dash L (aka, 66L) attempt. Grimnir has superb reach with his f5H, and he has projectile that can either be fired off or stalled (instead of firing the tornado off, make it stay in place in front of you and watch your opponent dash into it if 66L is all they do), with modifiable trajectory too.
Being mindful of your opponent's 66L reach and stand just outside of it while poking with your long reaching attacks/projectiles can help.

If you're already within 66L reach though, playing defensively can help. Block the 66L first and watch for the follow up. Since 66L is plus on block, you don't want to try to punish right after. Keep watch for what your opponent does next. If they do another 66L, feel free to keep blocking until they do something else. They have no way to open you up if this is all they're doing so eventually they'll have to perform a different move.
That is, if you don't want to spend your own resource/don't have the resource to spend yet.
If you do though, don't be afraid to burn BP for brave counter or use your ultimate DP (forward + U skill). Don't forget that Grimnir has superb mixup that's very difficult to deal with after setting up your crests so there's nothing wrong with burning some of your own resources to steal the turn from your opponent in order to setup your own offensives.
Meter also builds up pretty quickly in this game so if the opportunity presents itself, don't be afraid to spend your meter for that ultimate DP.

[Unrighteous]

Just to make something clear, you can't punish 66L.

66L is +2 on block, so the person doing it has the advantage. If they're spamming 66L over and over again, you can interrupt it because it has a long startup, but you're always at a disadvantage when blocking it.