Well, I'll explain as best as I can:
A lot depends on the map; if it's small, you need to focus on upgrading the right units to clear out the neutrals. Also, prioritize luck, archers, and knights with wind magic (their high defense will be helpful at the beginning), plus some druids and a high-level unit (if you have enough wood)...
Besides luck, you should have attack and ranged attack, and preferably some light magic (chaos isn't very effective). (Stone Skin or Haste – both are mass buffs).
In longer games, it's different; it's better to build ents and dragons as quickly as possible and take gold from chests more often so you can upgrade your castle and capitol.)))
And, of course, hero development: if you have a mentor, you don't need to worry about missing out on skills, but otherwise, be careful when choosing them....
And I almost forgot: single units, and again, single units – they are our everything in the early stages... If enemy units can reach our still-weak units, you can count on losses, but single units will help prevent that!