Rexaar

The baby dragon must be in your first pet slot.

Mana battle give 3 box when you win, and you can do it 3 times per day per world. (probably 1 box if you lose)

JMS patch note say

https://maplestory.nexon.co.jp/notice/view/950722/?id=all&p=2
Every time you defeat 200 monsters, a youkai who is taken prisoner appears with a certain probability

You can only enter the mana shrine 3 times per world. You may have miscounted how many times you already entered that day.
Mana shrine portal has a 5 minute cool down between appearance if the kill count requirement is met very fast.
Killing 25 mobs every 5 seconds will typically spawn the man shrine portal as soon as 8 minutes. (regular spawn rate)
Killing 25 mobs every 3/1.5 seconds will typically spawn the man shrine portal as soon as it come off cooldown. (kishin/wild totem/frenzy)

Has to be a real genuine enhanceable Absolab (already mentioned in the patch note).
Cannot be a fake unenhanceable Absolab acquired from any events.

Having pet heal at 90% is dangerous in most bosses with potion cool down as because every hit will activate the potion cool down and you will find yourself dying because you can't heal when you need it. 50-60% is more reasonable when fighting with a potion cool down.
Although having auto potion in the higher difficulty of magnus is not optimal due to potion only healing 10% outside the blue barrier. (In easy mode, potion only heal 50% outside the blue barrier if you are not zombified).

Staying out of range of Magnus spin attack is your first priority.
Dodge any attack that take Magnus 4 seconds to charge up, they hurt alot (straight up kills you at normal/hard)
Dodge the meteor if it is humanly possible.
Magnus (and most bosses) is at their most dangerous when they are walking toward you because they can attack anytime and you can't dodge if you are in an attack animation. It is safest to attack the boss during their attack animation.
Standing in range of Magnus spin attack with a meteor behind you is suicide.
Pay attention to Magnus web debuff, it does no damage but it web debuff slow you and prevent you from using any mobility skill.
Anima Hero's Will will cure zombify and web, but knowing when to use is a difference of 1-2 life on average.