I also think it's likely this Mark X isn't used to fighting serious competition. The name Executioner implies an expected power imbalance - it sounds intimidating, but an executioner's job is to kill something that's already been made helpless. This is not a droid built to be used in any kind of a fair fight

It's basically the droid equivalent of Jabba's rancor, but it also comes with a video game setting so that Hirken can get the immature fun a child gets from blowing through a video game on the lowest difficulty with all the cheat codes enabled. Someone playing like that is absolutely not interested in anything resembling a real challenge.