To clarify why Realo isn't making good suggestions:
As a tank, you want to hit about 200 Armor/MR's worth of mitigation, AKA 66% pre-reductions. This makes it so that, even if the enemy packs LW or Voidstaff, your effective HP is double the listed value. Leona has good base armor and has W, so she needs...I want to say 60 armor to have optimal armor, then either 110 or 85 MR. In other words, if you are buying a Wits End, an item that is purely resists and unreliable resists at that, from a defensive standpoint, you're doing it wrong. Banshee's and Mercs gets you to the 200 MR threshold more or less: you do NOT want more MR than that, as it provides diminishing returns. As for Armor, again, and this is important, buying armor just for the sake of armor leads to failure: it has to provide a great side benefit to be worthwhile. Randuin's is a good purchase because it lets you skull**** carries with the active. Sunfire isn't good because it provides puny damage and is stat-inefficient. If you have a choice between Sunfire and other items, go Warmogs and Atma's if you want deeps: it synergizes with triforce better, gives you actually scary damage (180 damage with sheen procs and 30% crit is no joke), and beefs up your defensive stats to the point you won't melt unless the enemy team focuses you all day long.
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