You get over heat as long as you are in the sun - head for shade
Kill 10 mulmen - buff elite appears = buffs all the mobs
You kill 20 mulmen - Elite appears
Once you kill the elite, you have 60 seconds to take out 20% max of the bosses health
Rinse and repeat! There are fruits that give buffs, and lots of other things to watch out for - like the electric balls etc.