Heros lv 20

You can get heros level 20, but if you do that, you won't be able to pick all the heroes. The strategy is really good if you are active and rotationate your heros between your cities. Having a Medea lv 20 for your slinger nukes, will hit like a harpie nuke, because the boost  is very high and having a Chiron, you spend much less silver and stone to train hoplites in a very short time.
1155 coins are needed to upgrade a hero from lv1 to lv20.
These heros in max level will be the best ones:

Ajax(35) - This hero will make your hoplite nukes very strong. Assign to your city when you need him for a hop attack. Don't forget to make more hoplites than catapults(in a revolt wingorld), because he works better with hoplites.
Andromeda - Well, you'll start with her. She is really good for city development.
Aristotle(105) - With him, your LS production surpass the birs production of the enemy. Cheaper and less time to be built. Make a tour with him in all your LS cities. It's a good idea to have max warehouses in your LS cities, so you build LS only if Aristotle is assigned to the city and you don't lose resources.
Atalanta(105) - CS attacks are faster and more effective. With her, the enemy may be surprised by a fast CS attack. Save her for all your conquets.
Chiron(105) - Now hoplites are cheaper than slinger and they take less time to be trained. Train your hoplites with him.
Leonidas(110) - A strong hero to defend revolts. You may recruit him or not.
Lysippe(45) - Recruit her when she appears. Your horsemen nukes will be so strong as harpie nukes.
Medea(40) - She is insane with slinger nukes. She would be my first heroine in lv20. Use her in your slinger attacks. Don't forget to make more slingers than catapults(in a revolt world), because she works better with slingers.
Zuretha(95) - She will improve your LS attacks, even though she dies in every attack. But save her for attack.

My priorities would be Ajax, Medea, Aristotle, Chiron, and Lysippe and
Zuretha, Atalanta, Leonidas and Andromeda
The first four ones have the highest boost and will help you a lot. With them, you have powerful slinger and hoplite nukes, along with LS nukes.
Then you upgrade the rest. As you see there are more Heros of War than Heros of Wisdom, so you should focus in red coins. If you finish to upgrade the blue heros, then Pariphaistes and Daidalos are good options.

Maximizing the efficiency of each hero

One thing you can do to minimize the effort, but keep the same results is filling your training queue(2 or 7 slots, if you use the premiums). All you have to do is send all the resources of the cities of the same group(LS cities for example), then you send wood and silver to one single city and fill the queue. After you finished it, the city will be full and you are free to send your Hero of Wisdom to another city. Examples:

Aristotle
Choose a LS city, assign him to this city and cast Call of the Ocean. Make sure to build LS only if your warehouse is full of wood and silver. After you finished the first slot, then send wood and silver from all your LS cities and make another slot. Repeat the process until the queue be full. By doing this, you don't need to train every day. Depending of  the world speed, you have to wait one week.

Chiron
Choose a Hop city,assign him to this city and cast Population Growth. Make sure to train hoplites only if your warehouse is full of silver and stone. After you finished the slot, then send stone and silver from all your hop cities...

Medea
Everytime you need to make an attack in a city with wall lv25 plus tower, use her in your slingers nukes. You'll probably lose all your troops and she will die too, but the damage will be so high that you'll need only more one nuke to take down the city and put a revolt. She die but she can be healed, and after that you assign her to another slinger city.

Ajax
Similar to Medea, try to send both heroes  together, behind a LS nukes of course.

Lysippe
Similar to Medea and Ajax, try to send both heroes  together, behind a LS nukes of course.

If the hero of war dies(probably you lost many OLU or all of them), then assign him/her to a "fresh" city. So when they woke up, they'll be ready for battle, and don't forget to rebuild your army in the others cities.