How do I survive the opening of Jagged Alliance 2: Unfinished Business?

I am not sure about how UB starts (and I cannot find a map of the Area), so I give you some general combat instructions for JA2.

Climbing up a roof is always big help. So if there are houses with flat roofs in the area use PAGE UP or just click on the Up Arrow in your GUI to climb it. On the roof you can crouch to the edge, you have to figure out yourself when to be in what position. For enemies far away you can lay down around two fields away from the edge (and move backwards I think using Alt+Click) for enemies standing very close to the wall you might have to stand and move directly to the corner to be able to see and shot them.

Use Z to sneak, your path will turn yellow if you do so, this costs more AP but causes a lot less noise.

Noise in general is a very very big factor. Enemies hear your gunshots and come towards you, so do not concentrate your mercs at only one spot, try to lure the enemy towards one or two of them and ambush from the sides.

If you are overwhelmed by enemies, try to retreat, that mean run away, recover, go around where you think the enemy is and atack again. Do guerilla tactics, hitting the enemy once or twice might cause enough bleeding damage to weaken them more or even kill them.