: No tricks I know of.
: Just run up the stairs to the two niches left and right of the entrance, hide
: behind the pillar when he shoots at you, shot at him with whatever you've
: got when he comes up the stairs, jump down, run to the other side and
: start over again. Repeat 'till he's dead (takes some time). It’s hard.
Mordred is immune to all weapons except the Excalibur. The only way to defeat him is to use that sword. You are right in that this is probably the toughest one on one battle in the game. While there is no recharger available during the battle, there is a hidden 2x canister within this room. I won't spoil it and reveal where it is. But it definitely comes in handy, especially when playing on Champion setting.
