Lion, the guardian with different personalities, it can be a great help to you to win your battles with less loses, how to use it and how you can improve it?
After unlocking lion palace inside the castle you will find 4 characters, Gentle, Energetic, Fierce and Cruel lion, every lion add specific buffs to your attributes, let's check it out!

I-Lion Characters:
1- Gentle: This can be helpful to gather resources much faster as he boosts your gathering ability, it might help you during gathering phase of the gold event.

2- Energetic: This can be useful to attack dark monsters in order to get more scores and then topping dark monster event as it makes your troops more strong against the dragon, but this lion buffs won't apply against devil befalls.

3- Cruel: This can be never ignored when attacking someone's troops whether if it was castle, gathering, throne, towers or magic mine's troops .. This character is highly recommended to win your battles with less damage to your attacking troops, also this lion is required to attack devil befalls to get more scores.

4- Fierce or Lion Spirit: Defensive characters you can use it to defend your allies, defending rally in your castle, it only works in castles which means it buffs won’t be applicable in towers, throne, resource towers and magic mine battles, and please never let your troops in the castle without Lion inside, your troops will be even weaker enough to be killed by a weak attacker with lion in his march, so if you have extra troops in your building while your main lion is outside don't hesitate to activate your Lion Spirit besides your lord equipments, spending gold on it is worthy. ( Lion Spirit has same skills as Fierce lion)

II- Recommended setup:
It depends on what situation you want to use your lion for, but for battles it might be a little bit tricky.
Well, by taking lion your lion palace to level 27, you will be able then to use 6 lion skills at the same time, for example, if you are an archer player then I would highly recommend you to use:

A - For attacks:
1- Guard Block Damage.
2- Archers Attack Speed.
3- Crossbowman Damage.
4- March Capacity.
5- Revive.
6- Heavy Cavalry Damage.

Notes:
i- Use Cruel character.
ii- If someone strong in your alliance lunched a rally strong lion, then use lion king so that your troops can use his lion attributes to bettering rally outcome.
iii- The 6th skill can be changed according to the battle situation, if your opponent is having high amount of archers then you can keep the same setup considering you sent some heavy cavalries in your march, but once your target has more Cavaliers then it will be much better to use Spearman Crit with some spearmen in your march, because his Cavaliers will dodge your guards and kill your archers, so spearman will be good choice to save your archers from being killed by killing your opponent's cavalries.

B- For defense:
1- Guard Block Damage.
2- Archers Attack Speed.
3- Crossbowman Damage.
4- March Capacity.
5- Spearman Crit.
6- Heavy Cavalry Damage.

Notes:
i- Use Lion King if you are sending your troops to someone who has higher lion level and skill, you can find that by checking your reinforcement troops.
ii- Only use this character if you are defending castle building, otherwise use Cruel if you are going to defend in the Throne, Magic Mine or Towers.
iii- Revive has been removed because while defending, the defensive troops will not die but only wound unless if you exceed your hospital capacity.

III- Ways to upgrade your Lion:
There three ways to upgrade the lion.
1- Upgrading Skill:
This is the easiest way to upgrade lion skills, you only need to claim a certain amounts of Assault and Defense points to get your skill up, how to get them?
You can get them by killing someone’s troops whether by attacking or defending, for that I will recommend you to better participate in Throne, Towers and Magic Mine battles, the battles in these buildings give you double Assault and Defense points.

2- Enhancing Skill:
When you do this upgrade, it will trigger your lion strength and so your troops ability to be more effectively in battles, you only need to get specific number of lion shards to upgrade your skill to next stage, there 6 stages:

Common: 0 lvl
Uncommon: 12  lvl 1 shards
Rare: 132 lvl 1 shards
Epic: 1332 lvl 1 shards
Legendary: 13332 lvl 1 shards
Mythic: 133332 lvl 1 shards
You can get these shards by participating in some event, such as: Gold event, Treasure Trial event, Ceremony Event, Devil Beffal event, Mammon's Lure and Monthly sign in.
Also dont forget to join COE website to claim shards, COE offers you 30 free lion shards everyweek, you can find it through this link: https://coe.leme.hk.cn/m

3- Lion Experience:
This can be done automatically without any focus on it if you are already walking after Lion skills.
Getting lion experience helps you to upgrade The lion level itself until level 60 (Max), you can get it by killing others troops, the more you kill the more experience points you obtain (Report above), and again fighting in Throne, Towers and Magic Mine still the easiest and fastest way to upgrade the Lion for both free players and spenders.
Also, you can obtain lion experience freely by upgrading Lion Throne to level 30, it will give you free experience every day, so don’t forget to claim it.

You can obtain it also by gathering resources in kingdom plots as well as alliance super mine, killing monsters or by participating in Gold event, Treasure Trail event, Devil Befalls, Dark Monster and Expedition event as well.
What does this really do is triggering castle attributes, let's take as an example the Cruel character: Troop attack, Troop health, Troop defense.

These bonuses is one of the most hardest stats to upgrade in COE, you can get them only with few ways like: upgrade VIP, upgrade Glory Badges… so working hard on lion will help you to improve your attributes very well and makes your troops even stronger.
However if you are interesting on spending on your lion to make it much stronger in no time, then Gold Top-Up event might be for you, in this event you can upgrade your lion with Skills, Assault and Experience at the same time, have a lock on the rewards!

Finally, Lion can be your key to win your battles or losing it, so knowing how to use it wisely will save your troops and makes the game easier to you to become a strong player in your alliance or even in the kingdom, Lion is only one feature in the game which means you need to use it besides another features like heroes, how you use them together will decide the result of your battles, and for that reason I would like to recommend you to have a look on heroes feature through the following link to make your lion more deadly: https://coe.leme.hk.cn/m/newsdetail-731.html
I hope this was helpful, and Good Luck!!