On most tables, my tactics are just to keep the ball in play and try and hit things that are flashing.
I usually look up one of the guides to see what the launch skillshot is, as some of them are quite straightforward (I can get the Portal one quite reliably. Never managed to get the Adventureland one though).
I know the Portal table better than any other, so I do have a couple of other tactics on that one. I like to choose the Sentry Turret mission first, as that is a multiball, which helps activate other things on the table.
Then I alternate between that and the Ratman’s Den mission, which is a mini playfield I like to use as a break from the main table.
For my highscore I didn’t manage to complete any missions. But I activated plenty of multiballs that got me entry to the turret factory mini playfield (which you unlock by hitting the bumpers enough times, and gives up lots of points), and unlocked an extra ball (don’t actually know how I did that!).