Yes, assassins are meant to counter mages.
Yes, they're doing it a bit too well because of how strong MR itemization is for them.
No, mages do not need more survivability.
They are not supposed to have it.
They have CC in their kits, inherent range, AoE, and multiple health option items for a reason.
They are supposed to stick with their team.
**No, an assassin shouldn't be able to tank a mage's abilities.**
**No, the mage doesn't need more armor to help tank the assassin's combo.**
They're not supposed to. If they get hit by an assassin's full combo while Zhonya's isn't purchased or off-cooldown, and they weren't with their team, that is their fault for being out of position and they deserve to be erased.
**Building full MR style on an assassin guts their damage, but it guts the mage's damage against them as well, but it guts the mage's moreso on average.** 250.46 MR on level 18 Zed with standard MR glyphs and a Guardian Angel if he goes the MR build path versus 141.2 Armor on level 18 Annie with standard Armor seals and Zhonya's. Zed's MR build will only have Edge for penetration whereas Annie has Sorc Boots and Void Staff. **Zed's damage will be reduced to 44.2%** (45.3% if you count Precision, assuming he doesn't have Lethality runes). **Annie's damage will be reduced to 40.3%.**
That means neither of them will do that much, but the assassin (who can use autoattacks well whereas the mage can't as they rely pretty much solely on abilities, as well as having lifesteal) will just beat them to death with a blunt stick, even if it takes forever. (Yes, the mage being alone or their team not helping for that long is a separate issue).
# An assassin shouldn't be some tanky duelist with pansy-ass damage that just eats everything coming to them. They should be glass cannon but burst down their targets just as fast as they would be. Their targets also are not supposed to be tanky against them either (unless they feel like sacrificing a helluva lot of damage).