My own distribution is like that
Fireball 15
Firewall 15
Flameground 3
Hellbreath 1
Firepillard10
Enchant Fire 1
Because the damage is = to spell damage + Your Magic attack, the little you gain from the lvlup don’t matter this much late game (Still i’m not soo sure with those new monster and their insame Magic resit XD)
What you whant from lvling your spell is more damage tick / cast
And in case of firepillard, A bigger AoE is alway nice.
I haven’t lvlup flameground that much because it only 1 sec per lv and it start with 15 sec that I consider plenty of time.
Also Agni mutiplier on flameground is around 1,6 (+160% or 260% in total)
While firewall is 2,45 if i remeber right. (345% in total)
It don’t mean lvling flameground won’t be usefull because the extra second won’t be lost later when the monster become WAY more tanky (R8…)
But the choice is between Flameground and Firewall.