You can switch to another weapon to use your shield, just don't do any damage with the other weapon.
You can then throw your arm ( cn_O ) at the Stained Glass Demon's heart when it is exposed to do damage. On the level The Hilltop Mausoleum, you must work through the level to the end of it, then head into your inventory with touchpad and select your arm. I couldn't get a hit in with a melee weapon, although it may be possible. You can however use any ranged weapon you wish for this. I will copy my guide over for how to do it armless.
The Not So Armless trophy in MediEvil worth 21 points Defeat Stained Glass Demon with just your arm.
