How important is safety? If an accidental "firing" is no problem, you can make the engagement short and the release quite light. (My blood sugar test "gun" has plastic trigger and sear and a light "push", but if it goes off too soon, there is no problem, just re-cock and try again.)
Also, polish the engagement surfaces and use a true 90 degree angle on the "cocking piece". Most guns use over 90 degrees so that the sear/trigger actually has to push the hammer or cocking piece back against its spring. This is a matter of safety, but of course makes the trigger pull harder.
Let us assume that lubrication is not practical, so friction has be reduced in other ways. Ensure that the "trigger" spring is as free from drag is possible. If it is in a hole, make sure it doesn't touch the sides or if it is around a guide make sure contact is minimized, possibly by using a cruciform guide. If a stainless steel flat spring can be used, it would probably be better than a coil spring.
All FWIW,
Jim