If i use Karn, The Great Creator to turn a planeswalker into a creature then use Red Sun's Zenith would it be exiled, or just go to graveyard since it is a temporary effect?
Red Sun's Zenith's effect providing that if "a creature dealt damage this way would die this turn, exile it instead" is an effect that modifies the rules of the game (C.R. 611.2c), so that the set of objects that effect affects is not fixed, so that it will exile any permanent if it would die "this turn" as long as that permanent—
is a creature at the time it would die, and
was previously dealt damage this turn by Red Sun's Zenith when that spell resolved.
Thus, for example, if Red Sun's Zenith deals damage to a creature, then that creature stops being a creature, then it would die, then that permanent won't be exiled due to Red Sun's Zenith since it isn't a creature at the time it would die.
Thus, for example, if Red Sun's Zenith deals damage to a creature, then that creature stops being a creature, then it would die, then that permanent won't be exiled due to Red Sun's Zenith since it isn't a creature at the time it would die.