When playing through Campaigns, level up the hero who will move on to the next map as much as possible. This means maxing out their level, building magic level 5 in all cities, and having that hero learn it. Also, go through all the places that increase stats – strength, agility, luck, and endurance.
Don't be lazy and prepare two (or at least one) secondary heroes who will also move on to the next level according to the scenario. They also need to be leveled up as much as possible – stats, magic, and skills. Magic is especially important! Never move on to the next level without learning all available magic. These secondary heroes will help you at the beginning of the next mission with resource gathering. If they have good magic, they will also be useful in battles with neutrals and for defending castles.

Using this tactic, I easily completed all the Campaigns of The Restoration of Erathia, The Blade of Armageddon, and The Breath of Death (the most awful expansion, with stupid and monotonous Campaigns – you just go around collecting artifacts). And I completed the last mission in the campaign about the sword in just 10 days – thanks to the city portal.