Due to how there is an EXP penalty when you kill monsters far from your level, players who wish to optimally level are forced to kill specific monsters. Of course, this is a kRO Zero mechanic which some people like and others don't like. The problem though is that kRO Zero has a level cap of 120, while Ragna0 has a level cap of 99. This means that monsters that are Lv. 100+ like many of the monsters in Glast Heim, Magma2 and Niffleheim are not viable options to kill for EXP due to the huge reduction of EXP gained. Some monsters, like Loli Ruri (Lv. 109), Raydrics (Lv. 115), and Wanderers (Lv. 120), Tengu (Lv. 120) are so high level that even at Lv. 98, the player is still getting a EXP penalty when killing these monsters.
Also note that these monsters don't even give a lot of EXP. In their written respective order, their base EXP is as follows: 4352, 3503, 5286 ,11044
Meanwhile, Harpys (Lv. 83) which are a lot easier to kill than the before mentioned monsters, gives Base EXP: 5385 (plz don't nerf Haryps lul)
My suggestion is to reduce most, if not all monsters that have a level of 100+ to 90'ish. These areas are difficult. Impractical in most cases to solo. Let us group together to do these areas without having our EXP penalized.