I included exhaust, yes, I did in fact factor in resistances and showed you the base damage before spell pen and that then a mage can achieve full spell pen for near or neutral damage
also, even with an adc being slightly resistant to attacks, Zyra would still do about 50 damage per auto attack and the 3% bonus damage is a mastery from walking into the bushes...
also, this entire thing is based upon the support's inability to heal. If for instance it was a soraka support, she could heal twice and use her ultimate and heal spell to account for about 30 or 40% of this but nothing is ever perfect
zyra can unleash her entire combo in a matter of seconds and for the majority of that, the opponent will be crowd controlled
for instance, let's say auto attack, ignite, exhaust, and then a full spell rotation, the damage is there. of course, that only accounts for the lower elos, but so long as there is a target (such as if caitlynn were to land a trap) then there is no reason why her full damage would not happen other than player inability
do some research on her stats and the masteries and see what you come up with. Of course an ADC should run magic resist runes to account for their low scaling, but some opt for outright damage
even so, if you account for 30% of the damage with resistances (if they were potentially optimal as i'm not sure what the exact format is but the player could achieve a decent amount by sacrificing early offense), let's say that leaves them with neutral amounts of base magic resistance total as the wikia seems to account for by stating the amount a total of 25 magic resist would account for though if we use their exact formula, 30 magic resistance: 100/130 = 76% of damage is dealt
now, let's apply that to my figures and bear in mind that is their optimal scenario unless they bought items early for hexdrinker
for the sake of saving some typing, i'm goign to include armor in these results (and yes, each auto attack is roughly 52 damage at level 6 from a zyra so with 2 that comes to 104 instead of my 100 estimate)
all of this in mind, it still comes to 846 damage,
with this in mind, it will take roughly one ability and one auto attack on the part of the adc or 2 to three auto attacks to kill the opponent
Soraka can increase that amount, but so long as the opponent is not given optimal circumstances, it is still almost a guaranteed kill with less than 300 hp remaining
corki, ezreal, varus, caitlynn, jinx and any marksman who has abilities to cast within their early game can finish this amount
optimal circumstances
if they have a soraka to heal or a nami to end the fight, then yes, the ADC will survive
blitzcrank can throw this math out the window as any opponent within range long enough is going to die
it's all situational, but yes, a support zira with absolutely attrocious gold income can finish off a marksman that has not specifically dedicated runes to countering such damage
also, within the build I set up, the masteries increase allied damage to the target by 3% , although minor, it can add figures