In Challenge Mode, Gregorovic gains increased combat stats. His life points, however, still remain at 200,000, unlike the other generals who are given an additional 100,000; however, his healing ability is likely able to heal him around this much. Unlike normal mode, Gregorovic in challenge mode is immune to poison.
Gregorovic's abilities are somewhat altered compared to normal mode and he gains a unique ability when he first falls to 0 life points.
- His "RISE, CHILD!" summons two spirits instead of one. In addition, these spirits move towards Gregorovic at running speed.
- Gregorovic's shadows are summoned at 140,000 and 60,000 life points, the same intervals as in normal mode. However, an additional shadow is summoned.
- When Gregorovic first reaches 0 life points, he turns the player into a shadow copy of himself while he starts healing himself with an initial set of 5,000 before healing for 10,000 afterwards every few ticks. The player will need to kill six Wight Hunters through auto-attacks (abilities cannot be used). Only food and Saradomin brews can heal the player - Soul Split does not heal from the player's autoattacks.
Players should fight Gregorovic just like in normal mode, as he still follows the same attack pattern. However, use of Soul Split is completely discouraged in hard mode, as Gregorovic's attacks deal far more damage at an increased accuracy rate which makes it near impossible to recover lost health from it alone.
Begin the fight in one of the corners, preferably the north-western corner so Gregorovic will only summon mania and delirium spirits. If a spirit of rage reaches Gregorovic, it increases his damage even further. It is best to simply ignore the spirits and let them reach Gregorovic; attacking them reduces the damage that could have been dealt to Gregorovic instead and because of their running speed and doubled numbers, they often reach him regardless.
Once Gregorovic falls to 0 health, he yells JOIN ME! JOIN THE COLLECTION! and transform the player into a shadow clone of himself. He immediately heals 5,000 life points before healing for chunks of 10,000 at a time. Gregorovic will yell EMBRACE THE DARKNESS., and several Wight Hunters will spawn and attack the player, using magic, ranged or melee depending on their combat style.
The player cannot use abilities or teleport in this state, nor can they change prayers via action bar keybinds. However, prayers can still be changed by manually clicking them in the prayer book. The player's autoattacks have 100% hit chance on the wight hunters and deal 2000-4000 typeless damage with each hit, with a chance to inflict poison damage; it also bounces onto other wight hunters, dealing damage equal to half of the first hit.
Lure the wight hunters close to each other to exploit the autoattacks' AoE. Eating food in the shadow form delays the player's attacks, subsequently allowing Gregorovic more time to heal. Drinking Saradomin brews while spam clicking on wight hunters lets the player to attack without interruption, however.
Once six Wight Hunters are killed, Gregorovic stops healing and says NO! ALLOW YOURSELF TO BE CONSUMED! and the player is returned to their normal form. Gregorovic does not restart his attack pattern; instead, he starts from where he left off prior to falling to 0 health at the time. Note that Gregorovic recreates four shadow copies of himself if he healed up to 60,000 or higher before the player killed six Wight Hunters.