You can't do them alone. They're capped at level 30, and the bosses are rather hard even with a full party. You can't fight it as an alliance either, but you can fight as an alliance up to the boss fight.

Your best bet would be to find a group of people that do these things at regular intervals. Depending on the strategy they use, you will be asked to bring various items, such as healing potions. You will also be asked to bring different types of Animas. The leader will probably tell you on beforehand which types of Anima you're supposed to bring, and when to use them.

My group wiped on my first try <,<.