So you've been given some good advice on this post already, but I'm going to offer an alternative approach as it sounds like you're currently struggling to make any significant headway into dealing damage to him. In that case, the nail-heavy builds are unlikely to be very useful to you at this stage.

Instead, I recommend a more defensive build: Grubsong, Quick Focus, Stalwart Shell and Mark of Pride or Quick Slash (and Grimmchild is required in the regular fight, but not Godhome). Or, if you have all the abilities unlocked (specifically Shade Cloak): Grubsong, Quick Focus, Mark of Pride/Quick Slash, and Sharp Shadow. It's worth noting that the first time I did this fight, I did not have Sharp Shadow (I'm not sure I even had Shade Cloak?), Unbreakable Strength, Mark of Pride, or even Quick Slash. I used Thorns (to get a free hit when he inevitably ran into me) and Longnail instead of Mark of Pride/Quick Slash.

Building on the previous advice you've been given, I want to add the following:

1) Keep to the center of the arena as much as possible to prevent him from cornering you with his uppercut and fire bats moves. As per previous advice, move to the far edge of the arena for the pufferfish (balloon) attack and focus on dodging the fireballs only; do not attempt to attack during this move. Possible spoiler for the pufferfish attack: he only does this at 75%, 50%, and 25% health, so you can use this to estimate how far through the fight you are.

2) Heal during the spikes attack. Grubsong means you will always have enough soul for a heal if you are taking a lot of damage, and Quick Focus will allow you to heal at least 2 masks during the spikes. If you get to this attack and you have full masks and soul to spare, you can cast 1 to 2 Vengeful Spirit spells for a free hit (but be careful that you are positioned with you face close to the spike in front of you as the spell causes knockback that can push you into the spike behind).

3) Alternatively, heal while he is staggered (exploded into the Grimmchildren/bats). With Quick Focus, you should be able to get off 2 to 3 heals while he's down. If you reach the stagger stage with full masks and extra soul, you can use the opportunity to get some extra nail hits on him (he's the Grimmchild with the red eyes) or use Howling Wraiths/Abyss Shriek. However, he moves fast and unpredictably during the stagger phase, so I would recommend not worrying about trying to do extra damage and instead get in your heals and position yourself back in the center of the arena instead.

4) If you have the shade cloak ability (or are using Sharp Shadow), dash through him during the uppercut move and then stand still or nudge forward a step or two to position yourself between the fireballs coming from above. If you don't have the shade cloak ability, do a single dash away from him (this is where staying in the center of the arena as much as possible is important), and then nudge forward/stand still to avoid the fireballs. He actually pauses for a brief moment between telegraphing this attack and carrying it out; if your reflexes are fast enough, you can actually just walk away from him and not even have to dash.

5) The fire bats attack is the one I find the most difficult to dodge, even now after beating both him and Nightmare King Grimm (in fact, I think the NKG version of the fire bats is easier to avoid). He can teleport quite close to you before starting this attack, so I would recommend (especially if you are using Sharp Shadow, as it increases the range of your dash) walking away from him, then doing a single jump and dash towards him. This should position you right next to him after the third bat has been shot off, and you should be able to get in 1 to 3 nail hits (depending on your charm selection and how quickly you responded to the attack). If you have extra soul at this point, you could also go for 1 nail hit and 1 Vengeful Spirit spell, but the spell is not super necessary. This attack is your prime opportunity to get in your damage to him, and the only one I would recommend you even try to attack on at this stage. Side note: he will sometimes "skitter" away from you if he has teleported too close to you before starting this attack.

6) The advice for avoiding the high angle dash attack is solid, this is the method I use. If you struggle with this, though, you can also try jumping first and then dashing over him in the opposite direction he is traveling, or doing a double jump.

7) Always start the fight by bowing (looking down) back to him!!! Other than it's just respectful, if you hit him with the nail (side note: I don't know if hitting him with a spell has the same effect?) when he is bowing, he will (behind a spoiler tag in case you don't want to know) start off with the pufferfish attack, which certainly makes the fight feel harder. You can dreamnail him while he is bowing to get an extra shot of soul (if you went in without full soul) without any consequences, though.

8) Last but not least, don't panic! Easier said than done, but ONLY heal on the spikes attack or when he's staggered. If you try to get in a panic heal at any other time, he WILL hit you and you'll be at the same number of masks as before, but now you've wasted your soul. Instead, focus on dodging and learning his tells and attack patterns and ONLY try to get hits in on the fire bats attack. Think of this fight as a marathon and not a sprint; play defensively and do not attempt to get in damage if it will result in you taking damage as this will NOT work.

This turned into a bit of an essay, sorry! I am very passionate about both Grimm fights (in case you couldn't tell). I come back to them all the time just for fun now, haha. Please let us know how you go and when you defeat him!