These three combos will cover the most common situations you find yourself in during a match. If you're new to the character and just want a few quick things to get you started playing, learn these, and you can branch out into other setups later.
(starter) > 214H > 2M > 236H > 5H > 623H > 22U (> 5U > j.6U whiff > j.H fuzzy)
This is the easiest fuzzy setup combo Grimnir has access to. Replace the 22U at the end with super if you think it will kill. If the opponent doesn't BC it's effectively a 50/50, and if it's blocked you can threaten a frametrap with the second ground seal. Can block after the safejump to bait BC; the subsequent strike/throw position is extremely strong for you.
autocombo or 2M > 236H~8 > 214U > 2H > 623M > 2M > 214H
Consistent midscreen combo, and the muscle memory will transfer well to later routes. The most common drop point is doing the 214U too late - you want to do it as early as possible, or the combo will often drop after the 623M.
2H (counterhit) > 236H~4 > run forward > 5HHH > 623H > j.22H
2H counterhit antiair is your biggest reward on hit. Also works off low non-counterhits. Note the weird 623H hit here; you get this if they're high enough, and it leaves them closer for better oki. If you're late on the 5HHH you'll get a clean hit and send them flying instead. Not a big deal, just means instead of getting oki you're reset back to neutral (with seals).
These combos are more difficult than the previous three. They serve as an entry point into Grimnir's other setup types. You should wait to learn them until after you've gotten used to the character.
autocombo or 2M > 236H~8 > 214U > 22U > f.L > 623L > 9U whiff > j.H fuzzy
The most popular of Grimnir's many midscreen fuzzy routes. Unfortunately more difficult than it looks - the timing for autotime fuzzy is quite finicky, so you often end up having to manually time the j.H to hit as late as possible.
(starter) > 214H > 2M > 236H > 5H > 623H > j.22U > j.99U (> j.26U whiff > j.L)
On the other hand this setup is *easier* than it looks: when you press U for the j.99U, hold the U button for the entire rest of the setup up to the j.L/2L. High safejumps, low loses to DP. Theoretically reactable, but this isn't common knowledge and it's a very difficult reaction so the absurd majority of the ranked playerbase treats it as a pure 50/50. Note that contrary to the slightly outdated video, on patch 2.00 the 22U now costs the full 50% meter (no refund).
(starter) > 214H > 2M > 623H > j.22H > j.3U > 8U > j.M > j.1U > rejump j.214U (> 236H~8 whiff, stop as soon as possible to set up in front of opponent)
And this setup is about as difficult as it looks. Threatens a very scary combo off of throw. Notably better against parry characters than Grimnir's other setups. The effectiveness of this setup is somewhat matchup-dependent, but when it works it really really hurts.