I've played 30+ seasons in Fenway, and I've found that it's tough to keep pitchers' ERA under 4.00.
Here are my two most successful pitchers there,
Jason Neill and
Lonnie Christman. As you can see, they were both really good in their primes, but they also threw up a few 4.00-plus ERA seasons in Fenway.
In addition to the obvious good splits, good control, good pitches part, finding pitchers with good velocity leads to more strikeouts -- which helps keep balls out of play. Good defense really helps there, as well. Finding one of those 120-plus inning SuA guys makes a tremendous difference in a high-scoring environment, because your bullpen is going to get gassed sometimes. But I usually just try to make sure my offense can keep up with the runs the other team is going to score playing half my games at Fenway.