This method should work for any race/class combination, but for some reason, Monk took me over 40 tries. As long as your Tav has 16+ Str, and Athletics Proficiency, this should work. You also want at least one of your characters to act after the Mind Flayer in initiative order so they can shove the Mind Flayer and force Zhalk to move, provoking at least one attack of opportunity. You can basically get a free attack on him every round. Also, yeeting the Lesser Imp on to the higher level usually ensures that it will remain alive, and keep the Mind Flayer friendly until the end of the fight.
One last note, if you kill the wandering brain, and then go back and recruit Us, you get an extra 10 xp. If you try to kill the wandering brain with Us in your party, Us goes hostile.