The key strategies summarized as following:
1. DO NOT level too fast
2. DO add every skill points on Hero Strength, despite saving a few for Stamina
3. Farm weapons and armors, do not buy them (unless you are generously buying in-game gems and coins with real money)
4. DO NOT add too many allies
For Advice 1 and 4:
The Battle List, where you pick your rivals and they find you, is working on a "equality-based" mechanism. That is, one always meet rivals with similar level and alliance number. A level 10 player with 40 allies could have all his buildings upgraded to level 3+ and a troop of 100+ advanced soldiers. But he could also be a noob with a small messed up army. And they would meet.
If you already added too many allies, remove some. (But not too much, or your will be surrounded by real nerds instead of meet lonely newbies.)
For Advice 3:
Every unit of your army could wear one weapon and one armor. Considering the size of an army increases fairly fast, buying them with coins is not a wise option. Killing goblins at the first map gives a very high drop rate of sword/armor sold at the price of ~10 coins. Later maps get better loot.
For Advice 2:
You gain XP every strike you hit on any monsters. Thus the more damage you do per hit, the less hit you would need to finish a monster, and the less XP you gain.
This is even more important if you need to farm for your army equipment.
Enough stamina is a must, and too much is a waste. From my personal point of view: 1/3 of level <stamina< 1/2 of level i.e. a level 10 player should have 3-5 stamina.
In addition to Advice 1:
Always try to buy the more advanced units, and do avoid the delicate ones.