Talk to that guy with the machine gun in the first map of Ulu City.
Finish all of the quests he gives you until you get the Mallet.
Get a Soul Latern from Captain Latanica (Basically if you haven't started the Captain Latanica questline in Singapore, you have to do that too before you fight Krexel).
With the Mallet and Soul Latern both in possession, you can enter the Krexel boss area.
But seriously, completing 12+ quests just to fight a boss with 500m HP that only gives ~4m exp isn't worth it. Bad drops too (except the occasional BS20).
All of the quest that lead you to Cap. Latanica, then Entering Ulu City from the same guy that gives you the quests for Cap. Latanica, then the guy he tells you to talk to.
But just saying this now, Krexel isn't worth it.
To start the quests.You gotta talk to the hobo looking guy(also in the basil banner up there nxt to search box) in boat quay town. Savior of Ulu city or something like that is the name. Ive done them all. Get a buddy to help.they are annoying collecting quests. Also like others have said. you gotta kill capt latanica for soul lantern. also a good source of exp for those lvl 95-105