Originally Posted by Panzerfaust
FYI, if you're going by the death recap, his auto attack shows under 'Toxic Shot' and is actually a mixed damage, if he's building AP then most of the damage is likely magic, if AD physical. It's one of the few abilities that you can't actually tell by just looking at it the damage type.
This is what is happening, luckily you can look at his items, if he has all AP items you can bet he's doing mostly magic damage, if he has all phisical damage items well get armor. It should show the damage seperately but than the game might count it as a casted spell, although it would give hilarious results with reylas it would probably be OP.
PS: in most champions normal builds you should have an item or 2 for defense, for example getting an atma's impailer for phisical damage or zhonyas ring for magic damage for getting magic resist, get a hexdrinker for phisical damage or abyssal scepter, upgrade for a purely defensive item if need be, if they have a fed phisical damager, you might need thornmail.