Looking at your question again, the above solution is only partial... it determines the minimum armies killed by X attackers (or inversely, the minimum required to kill N defenders)
The probability of each would require using a binomial distribution.
http://en.wikipedia.org/wiki/Binomial_distribution
For example, use 4 attacking armies and 60% kill rate, 75% luck. Here are your possibilities:
2.6% of 0 successful attacks.
15.4% of 1 successful attacks
34.6% of 2 successful attacks
34.6% of 3 successful attacks
13.0% of 4 successful attacks
Rounding errors give a 100.2% total.
Then take the expected outcome (2.4 kills) and multiply by 1-luck (.25).
Multiple the successful attacks by the luck figure (.75)
Add these two numbers together.
2.6% of ending with 0.6
15.4% of 1.35
34.6% of 2.10
34.6% of 2.85
13.0% of 3.60
Now those remainders are the odds of rounding up or down.
1.04% of 0 kills (2.6% * 40%)
11.57% of 1 kill (2.6*60% + 15.4%*65%)
41.72% of 2 kills (15.4%*35% + 34.6%*90% + 34.6%+15%)
38.07% of 3 kills (34.6%*10% + 34.6%*85% + 13%*40%)
7.8% of 4 kills (13%*60%)