Luck represents a bonus to Luck-based skills occuring (Pocket Thief's chance to steal, for example). It reaches an upper cap of 80% chance to occur, after which any further luck is wasted. Multicrit is the single most important stat in the game for your main tower, it represents a straight multiplication of Critical Damage. Dealing millions (or billions) of damage per hit requires high multicrit amounts (+7 or so). Each higher level of Multicrit takes a little more Critical Chance to achieve (Don't have the odds handy, but use the search function, they're on here). An example is 100% Critical Chance guarantees a simple +1 (Base) Multicrit, but might only have 80% odds of doing +2 Multicrit.