Leaders get a 2.5 factor bonus on their attack and defense. This makes advancing high rarity warriors to leaders a good decision, even when advancing them at high level consumes more XP than low level warriors require.
Attacks become more successful with a lower health of your opponent. Even with a defeat in the first fight, it is possible to finally get the kill with 3 or 4 subsequent victories.
As warrior's advance in level, their attack and defense increase one of two ways. I haven't been able to find any relationship between race, or rarity and the type of stat increase that warrior uses:
1. A linear trend that is the same for every level attained (given rounding to the nearest whole number). OR
2. Some form of exponetial trend that starts at low value and drastically increases as the warrior gets close to its maximum. See the explanation of experience required to level your warrior ; it is probably based on a similar calculation method.