Battle Stuff

So, I know every OK DM need some battles at some time or another, but really, it's not my strong point. But I do want the perfect monsters for each area. Most of the events do happen in the town/kingdom, so I want a monster that is easy to find and easy to negotiate with. Such as offering money. I was thinking of one battle where the Elder Twin (I still haven't come up with details on these characters, so bear with me) is trapped in the dungeon and the Younger Twin has managed to summon a Bearded Devil to guard it. (This is possible because the Younger Twin disguised herself as an Erinyes and used devil magic.) And maybe a dragon of some sort for one of the final battles. I'm still not entirely sure on other details but I really want my first campaign to be good. If not the greatest. I know I'm not going to create the greatest campaign in the world, but I want it to be the best it can be for a first campaign.