Replace any steel parts with Titanium or Magnesium parts, (depending on what it is). Don't know about on the Leon but things like intercoolers, radiators etc weigh quite a bit if made from mild steel. Replace mild steel exhaust with proper race spec stainless/titanium one (not SS for road use as they are still too heavy). As said get race spec (magnesium) wheels, brake discs and calipers.
Replace fuel tank with carbon one (if it's possible), replace all bodykit parts with Carbon or thin ABS. Replace all bolts with Titanium ones. Replace stock flywheel with lighter race spec version, (and get ready for some serious engine braking when off the throttle.
Of course all above is if you're serious about loosing weight because it will cost an absolute fortune to achieve.
cuprablue