Tanky sololane Leona - the strategy described in this guide - is a champion that deals mostly magic damage, has short range abilities and autoattacks, short cooldowns but 1v1 pretty damageless abilities, lack of strong waveclear, lack of mana (despite Doran's Ring and/or Manaflow Band she still remains mana hungry) and very specific "scaling".

As Leona, with this strategy, your main goal is to force 5v5 teamfights as much as possible. Leona is a very strong teamfighter - an extremely tanky champion with much hard CC who deals a decent amount of damage with AoE abilities and especially the Passive.
On the other side, most of your lane opponents will be champions who are strong 1v1, strong splitpushers but significantly weaker teamfighters, especially in the midgame. And even themselves can be very surprised how weak in teamfights they are compared to the Leona.
You want to use this fact - an easiest way to carry a game as sololane Leona is to just survive the lane, not to allow your opponent get fed on you, and just 1v9 the midgame by creating teamfights which will look like "enemy team wants to teamfight too", but in fact only the Leona's team wants to - only for Leona's team such fights are winnable and lead to the victory in the whole game.

Leona, however, doesn't scale to late game.
Her power in midgame should already secure the victory. If Leona's team doesn't get a significant lead during early-mid game and goes to the late game in equal or worse position, it usually means the game is lost for Leona, because she simply doesn't deal damage, her utility items start losing value and the enemy team will have one extra fully scaled "real champion", not just a "bit more fed support".

Sololane Leona is

strong against most assassins (Zed, Qiyana, Katarina),

decent against marksman champions without %HP damage spells (Lucian, Quinn, Zeri),

very weak against heavy pokers/traders (Azir, Anivia, Orianna),

gets outscaled and destroyed in later stages of game by splitpushers (Tryndamere, Nasus, Fiora) and

"works significantly worse" in lane compared to classic sololane tanks (Galio, Ornn, Malphite).


In many cases if you use this strategy, your only goal is to survive the first phase of the lane (like first 10 minutes) - I repeat it again.
In this survival mode, many "less common" things can help you very specifically against some champions, such as selecting Aftershock (for example against Aatrox it can be decent), Bone Plating, or the Doran's Shield (viable especially in midlane against some Oriannas, Azirs etc.).
Especially bruiser champions that are unfed are significantly weaker than Leona during the midgame fights she creates. Unfed Aatrox or Irelia teamfighting against "unfed Leona" = disaster for enemy team. Leona doesn't need the gold from the early game as much as these champs; she will still do her job.
And the enemy splitpush champions, despite they are long-term unstoppable, well, the speed of their push, resulting from what happened in the early game, of course impacts the win probability of your team. If you hardfeed them in laning phase, their push will take control over entire game as your team has to help you depush in such scenario. But an unfed splitpusher will still theoretically be able to push your inhibitor in minute 30-35, but by that time the game can be already lost for him.

If your lane opponent is an assassin, of course you have to try to keep him on your lane, but if he still leaves the lane and starts roaming, ping the roam as fast as possible so your team has time to react. Follow only those opponent's roams where you have a realistic chance to arrive before your teammates have been already cleaned up. On some occasions, even the Teleport can help you arrive to the center of action much faster - sometimes the Teleport can be used "preemptively" before there is a real fight just to scare the opponents and force them to either back out, or take a much less favorable fight for them.

Sololane Leona wants a teamcomp consisting of herself (magic damage tank), 1 support (preferably enchanter but it usually isn't that important), 2 heavy physical damage champions (at least 1 of them long range carry) and 1 long or mid range heavy magic damage champion.
Let's say something like this is the ideal scenario, because it ensures that Leona's team will have long range carry champions, will have the frontline tank (Leona), will have enough of both physical and magic damage.

In reality, a very big problem for Leona (for support Leona too) are "full AP topsides". Enemies in such cases rush Mercury Treads, often add Wit's End or Maw of Malmortius and then Leona just can't play - her team doesn't deal damage and Leona's CC is mitigated by all that Tenacity enemies have.
Solution... you can ask teammates not to pick full AP topside, you can try to build Iceborn Gauntlet or Abyssal Mask if a situation in game allows you to do so, but often just nothing helps and you'll lose (unless you stomp early-mid game).

Similar, but not necessary the same is the problem of tanky champions in general. Yes, most of the tank champs are magic damage dealers, but there are also few physical damage ones (Poppy, Skarner). And again - having too many tank champions can lead to big problems because at some point your teamcomp becomes pretty damageless and the game is lost if you lost early-mid game and have 3 tanks in a team.

Besides these teamcomp problems, Leona has one laning nightmare: getting to the state of lane in the early game where enemy champion freezes the minion wave close to his turret, keeps the lane frozen and starts zoning Leona from minions so she can't take any gold (and often even the XP).
The only defense against this (which happens mostly on toplane) is to try to keep lane frozen for Leona - if possible - and just not to randomly stupidly push minion waves when the opponent is on the lane ready to freeze them. But sooner or later the wave state will come to the freeze for an enemy so you simply have to call your jungler to help you crash/reset the wave.

On the other side, Leona herself "helps" her carries during the midgame in terms of resources. A tank Leona doesn't need much gold to be useful and doesn't even have time to farm every wave a normal laner would. Remember - on sololane tank Leona you want to create teamfights - that's your main goal. You often drop many minions just to be ready somewhere where you can initiate. Many times the minion waves start pushing towards your turrets (sadly often the enemy splitpushing champion stays with the wave too :D) so after you fight and somehow reset/respawn, the minions are ready to be farmed - and often your teammates - the carries - take these minions rather than you, and you're fine with that, as they need the gold, you don't (in most cases). It is a bit double-edged sword as your teammates waste some tempo time by farming something your role should normally farm, but in most cases the gold outvalues the map tempo, so "it is good to have a sololane tank Leona in a team".

This tanky sololane Leona is playable basically in any elo without need of "extreme changes" - just with little adaptations made for a specific game (teamcomps and elo), of which all are described in this guide.
"Ideal" is most likely Platinum-Diamond elo as too low elo means too long games and higher probability of getting outscaled and too high elo requires a skilled Leona player.
Personally I won like this even few SoloQ games with/against LEC players (!!!) so even in EUW Challenger such strategy is viable.

LimTheDestructor,
best Leona world,
support Leona main,
peak EUW rank 14 / Challenger,
peak EUNE rank 11 / Challenger,
10+ million Leona mastery points.

LoL-themed books -> https://allmylinks.com/limonite
special thanks to Raid Boss -> described: https://drive.google.com/file/d/1eFJ-jtB96HXwo4G5EAz5oly9iX5B3ohF/view ; pages 76-78