Blue is quite good for an in game trainer, and his Pokemon team, having multiple types, has no clear weakness. However, if one looks closer there are some flaws to take advantage of.
First of all blues team has three ice and three rock weaknesses it is smart then to carry rock and ice type moves. If you don't have either of these types, water types typically can learn ice attacks, and ground and fighting types typically learn rock type attacks. Speaking of water types, having a bulky water type on the team such as slowbro will be extremely useful in taking hits from blues powerful attackers, and with ice beam said water type can take on exeggcutor. Slowbro gets the mention because it hit machamp with its psychic attacks and has great defense. Also, blue's executor runs the move trick room, which causes slower Pokemon to move first. It is wise to have Pokemon that are either slow or don't mind moving last in order to deal with trick room.
Well that's it. Good luck defeating blue.