I think an one literary device that Golding uses to build suspense is the setting of the story. The setting is on a deserted island, and there are no adults around. The young boys are on their own. Readers also know that the boys are on the island because their plane was shot down. It wasn't a mechanical failure, so readers have the added worry about possible enemy combatants being on or near the island. Add to that the comment from Ralph that nobody knows where they are, and that rescue might be a long way off.
“The plane was shot down in flames. Nobody knows where we are. We may be here a long time.”
All of these details put the reader on heightened alert and add to the suspense of the story.
Another literary element that adds to the suspense is foreshadowing. Early in the story readers are told about a beast. This, by itself, adds suspense because so much is unknown. Is it real? What could it be? Where does it live? All of those questions are left unanswered, so reader imagination begins to take over. The frequent reminders about a possible beast foreshadow the coming conflict with that beast; however, about halfway through the story, Golding offers up a new foreshadowing warning about the beast. He has Simon announce that the beast might actually be inside each of the boys.
“Maybe,” he said hesitantly, “maybe there is a beast.”
“What I mean is . . . maybe it’s only us.”
A third literary device that Golding uses to build suspense is conflict. There is a lot of conflict in this book, but I am specifically referring to the conflict between Ralph and Jack. That conflict moves from each boy being wary of the other to a full blown life and death struggle. Their physical battles are suspenseful, and their battles to win the hearts and minds of the other boys are equally suspenseful because readers simply don't know which boy will win.