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 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.