Glen is fought multiple times throughout the story.
Chapter 1: Magi KnightEdit
You will be required to use VitaMint to restore your HP so be sure to equip one for this battle. Also, equip whatever Lapis you got from previous battle will help you a lot. Especially Fighter Spirit and Attack+5 Lapis.
Glen has just only normal attack for this battle so it is not much of threat but since he's faster than you, beware. Attacking from behind deals most damage and lower his evasion so go for it. Since you only need to lower his HP to 1/3, this shouldn't be too hard.
Note that you can get your very first material drop from Glen, the Rubescent Branch by first lowering his HP to about 75+ HP. After that, move behind him and use Cutting Blast with 2 DP to deal 75+ damages on him to completely defeat him. As Rubescent Branch will not be obtain until chapter 18, this is a faster way to obtain it. Keep in mind that you can get this drop when his HP reach 0.