The teeth and fangs started to come out after I left 30 and switched to WT1.
Going up... you can ride the wave if you keep up with gear and such but there's no way around this for me: This game is tough and my soloing has diminished quite a bit.
You *can* solo everything, but a few missions are murder. The one where you face three elite bosses at once (Sniper, Rusher, LG; it's one of the two in the SE quadrant) is murder solo.
That being said, your equipment goes up fast on average, so it should take just a few control points to get the requisite gear for each stronghold.