Stay out of - and well away from - the Plains until you're actually ready for the Plains is the best bit of advice I can give anyone playing this game. Otherwise you WILL get your butt handed to you. Discovered that on my very first play-through and I still get panicky when I accidentally sail too close or wander too far from home in early game stages.

Some weapons work better than others depending on which mob you're fighting. Mozzies are dead-easy to kill even when you've got lower gear as they're extremely squishy, but you have to be aware of them and you have to be fast. It's just that intial charge that'll get you. The trick to them is block and immediately swing. If you're using a slow weapon such as an axe or hammer, you'll have trouble. A silver sword works wonders on them. As for not being able to hit them easily if they're above or below you... well, that makes sense to me.