If you're talking about the military base part, on the way back to Yokohoma, then you need to make sure you're going into the right place.
While in the woods there, there are 2 exits. One that leads into the military base, it's like a brown square hole/bunker, with a ladder. That's where you need to enter. If you went into the teleportation thingy, then it warns you that the monsters are very powerful there. That's not where you need to go. And yes, you will die from 1 attack from those guys at that level.
Not sure if that's your problem, but if it is, hope it helps ya.