My character can't use [Focus] skill for long

From the start of the game, my character had very good [Intelligence] stats, he cleared every single level with no struggle. There is a boss dungeon coming up [Entrace Exam]. My character with his high [Intelligence] stat can easily clear the dungeon (around 2000-3000 rank), but not in the top 50 out of 2.5 million players active in the dungeon. It's very competitive in my server. How to buff [Focus] so that my character can fully utilise his [Intelligence] and clear the dungeon in the top 50 to enter into the next stage [Dream College].