Hordes from All Sides IV
A lot of field bosses are scattered in the area even though they have only separated into three groups. The core guild members or at least the ones that can stand up to them on a one on one scenario decided to step up. They needed to make sure that the horde of monsters do not get the buff that the field bosses give to those before them.
Unlike a regular dungeon boss, field bosses are somewhat weaker but what they lack in pure power is supplemented by their field boss buffs. The Sand Dragon that Basil went towards has a buff called Sand Evasion which increased the evasion of all the beings under it when they are in a sandstorm or in a sandy area.
It is not only Basil and Arsenal that started to move by themselves as others that can did as well. Since Adrian was the one giving commands of the guild, the others could rampage to their hearts content. Vayu, Solstice, Lycan, Soleil, Creepysoo, Apolaki, and Ark went to fight against field bosses on their own.