Batting Rook! Achievement
Promote to Rookie batting difficulty in Dynamic Difficulty.These sets of Dynamic achievements shouldn't take you more than a couple of games to unlock. The Dynamic setting is essentially a XP style level system based on difficulty. The scale can slide back and forth meaning you can gain and lose XP based on your performance. Like I mentioned in the roadmap you have to start out on Casual difficulty and select the Dynamic option to unlock these properly. If you pick anything else the game will start you out on a higher difficulty and you'll miss some of the easier dynamic achievements. Remember you are starting out on beginner difficulty so refer to the walkthrough for tips on pitching and hitting that will get you leveled up in no time. Keep on hitting home runs and striking out batters to progress.
It also looks like this achievement and Pitching Rook! (70G) are unlocking for each other (batting achievement for pitching and pitching achievement for batting) but as long as you meet the rank for both batting and pitching you should be fine.