Soldiers Grave is an intermediate grind-zone. Soldiers Grave mobs are primarily fought for quest-related reasons and also used as a grind spot early on. The xp here is decent but nothing special. With that being said, this spot is decent early on, but you should focus on questing until you reach level 56 - once level 56, you can grind Polly's for better, more profitable rewards. Polly's is also one of the best spots in the game for xp. If you reach level 56 but still cant kill pollys Soldiers Grave will do until then.
Soldiers Grave is located east of Heidel; Hediel is located South of Velia, the first major city in Black Desert.