The team I used for this was the Sirloins at level 62 and won against the Platypi with 12-3. Let me try to give some suggestions. I'm sorry in advance for the walls of text, but maybe some of these ideas will be helpful.
- To quickly start a new season match, when one goes really wrong, just start a completely new season, choose your team and spam
until the match starts. Every time your team will play in the first game of a season. That way you avoid waiting for the other season matches to be "simulated".
- Stats: I wouldn't suggest going for this at a level below 50, numbers matter. Try and have all of your team members with roughly the same stats. You need to feel confident in the capabilities of every single one. Focus on assigning your team members modifiers which improve their Power, Contact and Speed for regular team players, as well as Velocity, Junk and Accuracy for pitchers.
- Accommodation: First off, I suggest gradually bumping up the ego rating as you play the game whenever you get used to your current ego setting. Believe me, there's no reason to continue playing on an ego rating where you constantly demolish the AI. You need to get used to less and less aid from the game when batting, baserunning, pitching and fielding in order to have any chance to do anything on ego 99. I suggest the following ego path: 10 -> 30 -> 50 -> 60 -> 80 -> 99. Get really used to playing on ego 80 and you'll have a good chance at winning one time on ego 99.
- Tip #1 (probably): I'd recommend to not even bother going against pitching experts teams. It's really frustrating trying to hit their >90 mph pitches with incredible bends that fool you and arrive outside of your strike zone and make you hit them weakly. You really need to face a team that has poor pitchers to allow you enough time to decide whether you can get a good hit on the ball or you let it slide past you if its outside of your strike zone. If you can't hit the ball, you'll never win, simple as that.
- Batting: Do your best to not swing at pitches outside of the strike zone. Very often, intentionally, the AI will pitch balls outside of the strike zone hoping you try swinging at them, getting weak contacts or missing altogether. There are some slight patterns that you'll notice if you pay the game long enough. Observe the ball as soon as it leaves the pitcher's hand. Even on ego 99 you can get your batters walked, rarely, if you are patient and don't swing at pitches you don't feel confident about. Always go for power swings, even if it's more difficult to time them. Try swinging a bit earlier than you might be inclined to for possibly better results.
Getting homeruns from power swings will be your best bet to score points. Always try to position your swing reticle in advance where you believe the AI will pitch at you. Unless you already have two strikes, it's better not to swing at pitches that aren't directed towards where you pre-aimed. Try to make the AI pitch a lot of balls at you, as the match goes on they will get tired and throw slower and less accurate pitches at you and you should really capitalize on those chances and get good hits and homeruns before they substitute their pitcher. Hence why you shouldn't get discouraged if you don't score any runs in the first half of the match. And one more thing, don't bother with bunts you'll always get outed.
- Baserunning: Always try to push your runners to the limit and send them to the farthest base you consider they can reach before the ball gets there. If you see clearly that the ball will arrive at a base before your runner can reach it, always have him fall back to the previous base. In some situations, you can try fake running to the next base with a runner to make the AI throw the ball at it so as to cut you off, but then right away you back that runner off in order to give another runner enough time to reach the home base, if he's close enough. If you have a runner sitting on a base and you hit a fly ball to the outfield that you can tell the AI will certainly catch, begin moving that runner only after ball is caught, that way you might have enough time to advance a base or two. Base stealing is extremely risky and will usually fail against the ego 99 AI, so I don't recommend doing it.
- Pitching: It goes without saying that you must constantly vary your pitch locations and types. It is usually better to get as high a pitch rating as possible, aim for the circle. If you see the AI anticipated a certain aspect of your pitch, next pitch do something completely different. Always go for power throws. Avoid the middle of the plate and straight line pitches 90% of times. Do also throw some balls outside of the strike zone to fool the AI a little. In the second half of the match keep an eye on your pitcher's stats; if they become too poor, substitute him.
When an opposing runner is sitting either at first or second base, keep in mind the AI will sometimes try to steal the following base. Be on your toes in that regard after every pitch. Your catcher must throw the ball with maximum force at the destination base to have any chance of negating the steal. Also, watch out for random bunts from AI batters. Don't panic, pick the ball either with the catcher or the pitcher and out that AI at first base or double play if the timings permit.
- Fielding: Anticipating where fly balls will land right after they're hit is critical. Move your outfielders towards the destination of the ball right away. Jumping and diving with the right stick when needed is extremely vital to catching balls and outing the AI. Always try to catch balls that will land in foul territory, as well as wall jumping for balls that would be homeruns for the AI if they're close enough to the outfield fence. Do your absolute best to catch every possible ball!
When you have to bring a ball from the far outfield to the infield, use Relay throws with LT. Always power throw by holding down the throw button with the highest rating you can, even in the infield. Remember that if the AI has runners on your bases and you catch a fly ball on the outfield and the AI has less than 2 outs beforehand, you must deliver the ball to the infield immediately and stop those runners from advancing too much. Always consider double playing the AI if possible and you're sure about it, otherwise your best bet to generally out the AI is at first base.
An extremely risky strategy to out an AI when an outfielder of yours is holding the ball, is to fool the AI into thinking it has enough time to advance one more base, but while it's running, you throw the ball with maximum power at that base and the baseman catches the ball right before the AI reaches the base. Always look for the fastest way to get an out on the AI. For example, if you need one more out on the AI, with first and second bases loaded and the ball arrives very close to the third base, instead of throwing the ball to the other bases, simply move your fielder to the third base to out the AI.
- Luck: You'll need plenty of it.