Try saving any explosive weapons throughout the game and use them to beat the Demolisher.
Also, there are propane tanks scattered throughout the map during the final battle to help you fight the Demolisher. Use as much explosives as possible.
Lastly, we advise you to tackle the challenge in a co-op session as it might be terribly difficult, indeed, to win in a single-player session by design.
See also: The Bozak Horde - walkthrough