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Frank and his party had already explored the dungeon for about four and a half hours and had already defeated many monsters while exploring the vast corridors of the Dungeon.
As Cathyl and Gwendolyn gained the System, they had begun to level up the moment they slew a beast, and they were also able to gather experience points from the kills of others within the party, allowing them to level up quite fast.
Even though they had already built up some strength before having the System, they still started at Level 1 and needed to level up from that point onwards.
Even Gwendolyn who was a Rank 2 Magus was still at Level 1 at first, so she quickly leveled up all the way to level 4 in this time, as Frank had used his authority as the Dungeon Master to make the Monsters slightly more attracted to the party, to the point that they would come across monsters in every corner of the Dungeon.
Check "The Protagnist's Sister Is Actually the Strongest"!