Here are some not so obvious tips for the combat mechanics.
The leader's stats are only used to determine the skills. The troop type stats will determine the normal attack and the troop's toughness. This is true for the range and speed of the attacks/skills as well the it's damage. A high speed hero using skills can act very fast even if they are using slow troops. This means that you want tanky units in most cases since you are almost always using skills to fight. This is basically what I found after playing through one campaign, but let me know if this is wrong.
Different skills scale off of different leader stats. Magic is obvious, but the physical skills seem to have different types. I suspect that magical slashes and such might scale with something like (str+int)/2? I'm not sure, but some skills just do a lot more damage, probably because they scale with purely str. For example, Edge slash seems to be the most powerful early martial skill for damage/mp. Blade spin is absurdly powerful later on. Reading some comments on wiki states that million dagger scales off of speed, as does Assassination. Whatever the case, there are some skills that make everything else kinda obsolete. Keep in mind the range of some skills are higher than others. As for the buff spells, I believe these only affect the troops, meaning it will affect just the normal attacks, but I could be wrong.
As for the incredible power of Rakwaroth that was displayed in my run, I will try to explain. First of all, he starts with a field spell that can target buildings. This lets him farm lvls immediately. Just go to a boss city with lots of units and keep spamming tornado. His starting command and mana is quite good too. The skills he unlocks are pretty much suited for boss fights, especially the crystal and final bosses who have super high damage/defense/hp but very low speed. You start with absolute zero, cast quickness, and then absorb mana. Then you auto attack until the freeze wears off. You need to let it wear off because I don't think recasting it while they are frozen resets the timer. They will get one action, and then you freeze again, absorb, and auto attack. I killed most of the crystal guardian with his default druid type (since he can't get better units). I opted to keep the druid because they are tanky with high vit, meaning they can survive a lot of hits with just getting wounded, which would get healed back up with the ring of healing. Later on, I gave him vampires which do a shitload of more damage and are even more tanky.
Muriel is also a super carry, afaik. I didn't use her, but if she turns over to Hippa's side, she will get vampire troops without having to search for coffins. She also learns absorption. While she has no absolute zero, she has other defensive spells like resurrection to win by attrition. She has no field spell to farm lvls easily though.
Later on, pretty much anyone can be a powerhouse with the stat boosting consumables though.
