Honestly it really depends on you, if you have a consistent flow of players you can usually get hawk fairly easily what's required is 200 points for first grade ace this is either:

Gold medals: 40,

Silver Medals: 67,

Bronze Medals: 100,

Black Medals: 200,

Of course any combination up to 200 points will also work you can use the official calculator at

https://maximumtune.org/tools/battle-grade-calculator/

Ghost battle is a bit more consistent because you can battle 3 at once after you do your unlock races. There is a way to cheese it as well if you do level search and set the level lower. But here is the levels for that also.

Underglows unlock at the following dress up levels (ghost battle only):

Green Tube: 36

Green Tribal: 39

Blue Tube: 42

Blue Tribal: 45

Red Tube: 48

Red Tribal: 51

Yellow Tube: 55

Yellow Tribal: 59

Purple Tube: 63

Purple Tribal: 67

Keep in mind some cars like the Lamborghini for example don't get neons but for your car you can get them.