In this feat Rigby shoots a laser that can light up a monster that is literally a huge eyeball.
This shows that Peeps is spherical and heights at 2.47 meters, making him, in volume, 7.89023 cubic meters.
The average density of eyeballs is 1.1538 g/cm^3 or 1153.8kg/m^3. This means the monster weights around 9103.75 kgs.
The human body is, on average, 65% water. Eyeballs in specific can have an even higher percentage of water, so i'll be estimating the eyeball as water. Rigby, with his laser, can ignite the monster's eye easily.
E=m*c*ΔT. E = Energy. M = Mass. C = Specific heat capacity. ΔT = Change in temperature, in kelvins.
Since this is auto-ignition, and afaik there's no auto-ignition to water, i'll be using the necessary temperature to burn a body. That's 523.15 kelvins. Room temperature is 291.15, so the total change in temperature is 232 Kelvins. Now to apply the formula.
E= 9103.75*4181*232 E = 8830564670J = 2.11 tons = High 8-C. And Mordecai can tank this with only some disconfort.