3 Energist, 2 Vikings, and a Death Machine at the start, and all of them are slightly weak to slash. After some enemies are defeated, 4 more Vikings will show up as reinforcements.
The unit that you place in slot 1 (or manually reposition using "initial placement feature") is in the middle of your team. Having a tank in slot 1 can make it easier to group up the enemies so your team can easily take them out if you have good area attacks. You'll need to have a strong tank since all the enemies will end up attacking your tank with magic and physical attacks, and will end up in a chain.