well usually depending on what kind of champ you play you build armour or magic pen. Champions i usually recommend into these tanks are champions like zed, cassiopeia and all around with champions that can deal constant and sustained damage, this paired with void staff or lord dominik's regard ( the highest armour negation item in its respective stat page) you are going to be able to take them down quickly. Another champion who i personally enjoy into these tanks who build MR is void staff rush sylas instead of protobelt since chances are they are going to build MR into you, i hope this helps.
Tanks often build items base on their oppositions team comp and their carry. If their carry is an AP champion like for example Zoe then they will probably go for something like an Adaptive Helm , Mercury's Treads etc .To counter that you should rush for magic penetarion items like an Oblivion Orb and Sorcerer's Shoes One more thing is that you should pick champions based on your opponents team comp too like if they have for example Galio picking Zoe might not be the best strategy against him because of his built in magic resist sheild.
They play mid. Even assassins will have trouble with tanks (in most cases).
Cassiopeia is pretty good due to her DPS, but she's getting armor nerfs next patch so watch out for that.
Kog'maw can be played mid (some may see it as troll, so prepare yourself for flame). Does % target dmg, you can build him AP, but even if you don't he does a ton of magic dmg.
Brand is getting buffs either this patch or the next for mid lane and his passive does % health dmg.
Same with malz (his ult), but I heard he is very hard to play.
other than that you can go self peel and slowly wittle them down or build magic pen with Void Staff if they have more than 3 tanks. champions like Ryze, taliyah, lissandra (great assassin counter) all are good at kiting or self peeling.
Edit- forgot to add, Liandry's Torment also does % max health as magic damage, actually a really good buy on many champions.
malz is a pretty easy champ while brand is a medium skill cap champion and both of them can take out tanks with ease but if the enemies are still tanky consider taking magic pen(morelos,void staff)this will do the job!
Comment has been deleted
Second you can play
Cassiopeia is pretty good due to her DPS, but she's getting armor nerfs next patch so watch out for that.
Kog'maw can be played mid (some may see it as troll, so prepare yourself for flame). Does % target dmg, you can build him AP, but even if you don't he does a ton of magic dmg.
Brand is getting buffs either this patch or the next for mid lane and his passive does % health dmg.
Same with malz (his ult), but I heard he is very hard to play.
other than that you can go self peel and slowly wittle them down or build magic pen with Void Staff if they have more than 3 tanks. champions like Ryze, taliyah, lissandra (great assassin counter) all are good at kiting or self peeling.
Edit- forgot to add, Liandry's Torment also does % max health as magic damage, actually a really good buy on many champions.
follow ak521 answer!.