One trade and you'll have trait on deck, after that you gotta know how to make use of it, and his mobility and you are in, it is not that simple against a good deadshot but that's the concept, it should not be that simple for him to keep you out either if you are playing smart. If you can't get in at all you gotta lab this matchup a bit more.Dead shot keeps brainiac from getting in by stopping trait, stopping leap, stopping ia dk, bf3, and dash. If you honestly think that deadshot can't keep brain out the entire game then you haven't played good deadshots.
You described that once brain gets in the deadshot has to guess 5050 and if they guess wrong they'll eat a 500 dmg combo but that is simply untrue which was what I was pointing out
Once you are in, brainiac does massive dmg, you need one divekick and 2/3 mixups depending on the stage, or your positioning on screen and, that's what I'm saying, one knockdown and you should have your game stabilished, after that it is on you to make every hit count.
5-5 are supposed to be hard if you are playing good players, doesn't mean you can't make it hard for them too. Patience and good decision making are key to beat deadshot.