I know these type of posts have been fairly plentiful recently but one tip I would have for showdowns, especially the TA showdowns is to be patient with them.
I can't count the amount of times I drafted a team, played the first two moments, and just said screw this, skipped to the final boss, and lost handily.
The biggest advice I can give you is to play ALL THE WAY to the final showdown, maybe excluding the final moment because it usually involves a 3 inning game which by that time you should have a plenty good enough team to win.
The cool thing about showdown is that it gives you runs for the final boss after EACH moment you complete successfully! You actually SAVE time by playing each moment because it earns you runs for the final showdown.
Now granted, if you can't beat the 1st/2nd boss showdowns and never can reach the final showdown anyway, that's a different story and obviously complicates things. If you're really struggling against the first showdown, quite frankly, I don't really know how to help you other than just try to square up the ball more, I really don't know what to say.
This has really helped me, because I can complete a full showdown run and enter the boss showdown only down 4-6 runs (I usually fail at least 2-3 of the moments along the way), with 4-5 diamonds, at least 2 diamond perks and a fully gold or better lineup, and the whole ordeal takes me about 45 minutes, whereas skipping straight to the end, the whole thing takes me about 30 mins but with a MUCH lower success rate, whereas if I play all the way to the end, I almost always beat the final boss.
I hope I'm able to help some people with getting some more affinities done, as I know more and more people will be desperate to get more affinity points with stage 2 being announced. Let me know what your comments are!