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Good advice.
Unless you have infinite mana, you don't have too much mana. He was a forbidden archmage. You can imagine the amount of mp needed to cast forbidden spells.
Terrible writing! Or just plain lazy writing?
Yes. Millionfold in either quality or quantity.
Read till chapter 50. Only a day has passed. Mc keeps sacrificing body parts for a 30 second, 3 times boost in his power. He still hasn't explored this feature and the fact that only a DAY has passed....
Level ups?
They were nice.
The AOE not taking effect on him makes sense because of the Halo skill. But the Halo skill doesn't state defense from TRUE DAMAGE. That's why it's called true damage. If everyone could defend against it, it wouldn't be TRUE DAMAGE! What about ABSOLUTE TRUE DAMAGE(the SSS RANK SKILL)? It ignores all defenses, whether skill or item, dealing 100% TRUE DAMAGE on an enemy. It's literally on the DESCRIPTION of the SKILL!!! Here's a solution to prevent confusion for future readers; edit the Halo skill of the lord class player and add that it also provides immunity to true damage for that period and make the skill cooldown even higher, like 7 days or something(since it also protects from true damage). The above suggestion would make much more sense for the direction you want to take.
What do you mean approximately?