I summoned one And it has the graphic of a troll -___- Damn beta.

You need 10 essence (it's your maximum mana) mana regen will only fasten the rate at which you regain your lost mana points.

essence doesn't regenerate. Mana does.

Usually I level up my Sovereign in one of two ways- HP or Essence

I think its fitting that the Sov can increase these stats much faster than the basic stuff like attack and defense, however occasionally If I feel confident in my strategic position I will place a point in attack or defense or attack speed rather than HP or Essence, although about 80% of my levels put all points into one or the other.

(wether I am magical/need spells, or melee/need to beat on trolls)

... lol

Does the fire elemental have the same stats as a troll?

Does he have any flame powrs?

Well he has only 50 health haha so nowhere near as good as a natural troll.  I've noted elsewhere that they seem to desert if you run out of money