penalty for using unproficient weapon: -4
A lion has AC 15 (SRD), which would put the needed diceroll at 19 or 20, or: a 10% chanche to hit.
considering we have 50 halfli.. eh, midgets, that's 5 hits every round
(if we'd count the +1 racial bonus for throwing weapons: 15% and 7.5 hits each round)
the average sling damage is 1d3 (small) with a x2 crit,
A crit is scored in 50%*10%= 5% of the hits -> 3 damage
in the rest (95%) of the cases: 1.5 damage.
for an average damage per hit of 1,575
(or 33%*15% = 4,95% of the hist if we count the racial +1, for a total of 1,57425 damage per hit)
that would come down on a total of 7,875hp damage per round. (or 11,806875 per round for the +1 racial)
Lions have, by average, 32 HP, so on average, they would last 4,0634920634920634920634920634921 rounds.
(or 2,7102853210523529723148589275316 rounds with +1 racial)
with a full attack, a lion could at maximum kill 3 midgets each round (2 with claw (6dmg minimum), maybe one with bite (3 dmg minimum))
in four rounds, the lion would take out 12 midgets, leaving 38 to do more damage.
(or 9 if te +1 racial is included, leaving 41 midgets)
This leaves more than enough leeway (NL: speling) for the midgets to win the combat.
of course, it will take a little longer to take out the lion, as average damage is lost with each round, but there are still way more halflings left to compensate.
so, there we have it, statistical evidence that the midgets would win, and BIG TIME at it