It is soloable, but I gotta say this is definitely a game thats far more enjoyable with friends. And I say this as someone who generally likes SP games, or even some MP titles solo.

The game already scales hp/dmg with player count - so you are playing it in the easiest form just in terms of hp/dmg. Of course, it is actually easier with a friend to dispatch mobs with team work.

If you got the earth shaking raid, then your best bet if you are able unable to kill Trolls effectively is to lead them away. They will despawn when the raid timer is up. I know its no fun seeing your base get mashed up as you repeatedly die :/

You can get more creative with your base building to prevent raids (look up guides). Some also only kill bosses in seperate worlds, so the only raid type that happens in your main world with no bosses done is the basic one with necks and boars.