Put the character in lots of social situations that rely on charisma, and rarely let him use his custom Mech? The other characters will toss off all sorts of social situations they don't want to deal with ("you're our liaison officer, you handle it"), and the enemy forces will soon figure out his specialization and work to prevent him from using the custom weapon loadout.
Just make sure that he gets a battle or two with his custom Mech setup before you start sending the saboteurs after him. It might just be a simulation where he sets the new course record, and after that the saboteurs start getting contracted. They verify his skills by hiring a pirate unit to hit the planet, and keep an eye on his performance. Once they have verified his primary skills, they start getting to work.
Eventually you may have him engaged by Mechs with antilaser armor. Hope the character has learned other weapon skills and added some variety to his Mech's loadout.