I have a mountainous barbarian tribe that - for cultural and historical reasons - absolutely adores ambushes and surprise attacks.  They'll go out of their way to set them up.  They also vastly prefer melee combat over ranged.

**What melee weapon would be ideal for these barbarians to use?**  

I'm looking for something that would have additional benefit during a "surprise round" type situation.  If necessary they're willing to carry multiple weapons, one to use initially and the other the rest of the fight.

Technology is medieval Europe (~1500s) and the barbarians are on foot.  For the purpose of this question, assume they're ambushing units 50-100 well equipped and well trained soldiers from the same tech period.  They rarely have numerical superiority and will do hit-and-run attacks in those situations.