do all the effects and at the end of "between turn" count the number of damage counters, if there are more then the pokémon's HP, he's dead
look if he's dead after you applied all effects
meganium has 10 HP left, he's poisoned, +10, and his effect does -10, together that's 0
short: you can only be KO'd at the start of your/your opponent's turn.