Attack Tactics for Barbarian Kings
1) Damage Dealer/Spare Reinforcement
Because of sleep regeneration time, the most effective use of Barbarian Kings on attack is to deploy him after your main troops. Use other troops to receive damage while your Barbarian King deals lots of damage. Or keep him until the dust settle, then deploy him to pick off leftover buildings. These tactics keep Barbarian King unharmed, so that you can use him in every battle.
2) Tanker/Meat Shield/Damage Absorber/Decoy
Deploy him along with your main troops and make him your war leader! He’s going to die and sacrifice himself for your victory. This is necessary when you have to go all out to defeat those hard and challenging villages. You can also beneficially use this tactics if it’s the last attack of the day.
This tactic is not usually recommended because of you have to wait for quite some time before Barbarian King is back to full health again, especially at higher level, e.g. at level 10 regeneration time is 96 minutes.
Use Healer in combination of Barbarian King. Barbarian King is harder to die and becomes good tanker, also every time Healer heal Barbarian King, he will spend less time to sleep after end of battle.
Defense Tactics for Barbarian Kings
Unless you can put him deep in the middle of your base, he can be lured out to the corner and only 15 of Barbarians/Archers can knock him down so easily (same as P.E.K.K.A in Clan Castle). Better put him in Sleep mode, so that he’s always ready to use on your attack.
However, if you are away for a long period of time, you can put him on Guard mode for additional defense, by the time you come back to log in, he will be back to full health.