Couple thoughts. The game got much harder on 0.76 than 0.75 because leveling heroes was slower. I haven't tried 0.77 enough to evaluate it. I've mostly been played on hard, but I think the general principles will hold for the other levels as well.
Some heroes are easier to play than others. I think probably the most straightforward one to play is Magnar. His fire magic hits hard and at a distance. With Mangar you should be able to whittle enemies down to low level before they reach your hero. When using Magnar at the start, he and your other heroes should retreat at the start of each turn and then have Magnar cast FireDart -- this gives the maximum number of casts before the enemy reaches your line. You will need to what and have your mana supply recuperate between tough fights, although it won't necessarily take any longer than a fighter would need to recuperate to get his health back up.
Alternatively you can play a strong fighter. I've only done custom fighters, but someone like Verga is probably your best bet. (If you want to play Kingdom, simply custom recreate Magnar or Verga).
Generally Sovereigns with non-direct attack magic are weaker in the early game, so you may want to avoid playing them until you've got more experience playing. Although potentially Procipinee can be quite good if you can stack good enchanments on her.
Don't underestimate the importance of magic. For example, if you can unlock the "Book of Winds" tech it's Obscuring Fog spell will help a lot. Similarly Haste and Slow are both low level spells which can turn the tide in your favor when facing a tough battle.
You need to proceed cautiously. Only take down enemies with low hit points until your heroes gain a few levels. Don't proceed until your group is at full health, just sit tight until your health is back up. You won't need to do this after you get up a few levels, but early on, you don't have a lot of health margin especially for level 1 heroes.
I've found in 0.76 some of the starting quests found at modest inns are actually to tough for starting heroes. To get to the point where you can do these loot every treasure point you can find. Good gear makes a big difference. Take out as many single low-level monsters as you can. One butcherman should be pretty easy -- a group of three butcherman is likely to be deadly. Unfortunately under 0.76 it's tough leveling by killing these, but in addition to gaining a bit of XP they usually have a bit of gear which have a good chance of being useful to you.
Skirt tough enemies and go hunting for more heroes to hire. Loot treasure as you go along. If you're lucky you can get a band of 4-5 heroes and get them leveled up a bit and then come back and clear out those monsters that are preventing your settlement from expanding.