First off these are just my opinion on things and don't necessarily lead to the most fun, also they just apply to the game as it is now.
1. You should always aim to have combat characters be solo clear bad-asses that don't rely on mana to do their thing. Characters capable of clearing dungeons by themselves level much faster and can power-level non-combat characters more efficiently leading to far faster production growth. Mana reliance tends to lead to long downtimes or unsustainable crystal use.
2. The most important ability to secure early on is an all enemy damage source so the best combat races tend to be orc and fairy. Berserker gets a physical all enemy earth attack and druid gets a magical all enemy earth attack, neither requires mana. Other races get druid but only fairies get +8 mattack so they always do more damage for the same equipment and classes.
3. An often overlooked combat class is succubus. They can drain other's lust to give themselves a two day buff worth 15% to attack and mattack. This is huge.
4. The best healer is attendant. Bandages scale with target health and don't require mana. The ability to heal and attack is also important since if you take a whole round healing your effective healing is reduced by the amount of damage you take that round. This is much less important in multi-member parties but even then dedicated healers should use bandages when they can to conserve mana. Cloth is much cheaper and easier to get than energy crystals.