Yes, you do. But you can receive buffs!
The test is not simply slay any devil whose HD is higher than your own, you participate in a ritual battle against a summoned devil. You can recieve no external help during the fight, except for magic cast before the ritual starts.
According to the Path of the Hellknight Campaign Setting book:
Running a Hellknight Test: The Hellknight test requires each would-be Hellknight to slay a devil whose Hit Dice exceed her own. Such a test is typically performed to fulfill the requirements of the Hellknight or Hellknight signifer prestige class, which many classes can access by 5th level. As such, 6-HD bearded devils are the fiends most commonly battled during the test (higher-level initiates must face higher-level devils). Those taking the test are allowed to carry into battle any weapons, armor, wands, potions, and other equipment they wish.
Additionally, they may receive the benefits of allies’ magic, so long as it is cast prior to the test’s start. The ceremony preceding the test typically lasts over an hour, meaning that spells with a duration in minutes typically wear off before combat begins. During the test, should an armiger receive aid from outside the testing ring, she is disqualified and imprisoned (along with her conspirators). Such cheaters typically face a brief trial and execution.
More specifics on preparing and running a Hellknight test can be found on pages 44–47 of Pathfinder Campaign Setting: Hell Unleashed.
So, your character can recieve magical help, as long as that happens before the ceremony begins. They simply must have a duration longer than an hour, or your character will not actually benefit from it.
Example: Heroism cast by a 7th level caster should last 70 minutes.