Wu can indeed survive hits from smaller titans at 3-60 so if you are not planning on moving alliances anytime soon and your alliance is stable at 7-8* titans, you can definitly leave him there at 3-60
If that is the case then yes, Mist is definitly the best option of the others… I would do Wu Kong as priority next tho.
I wouldn’t bother with Sif for a little while yet… not until you have all the materials needed to take her through to 4/80…
Some general advice regarding Hero Levelling:
And a good thread on the vast benefit of Titan Damage Boosters:
I read this) but point about Sif - she is a counterattacker and can help me to finish many events (like costumes), when talked alongside maxed Boril, Melendor, Boldtusk and Rigard)
I do not have Cyprian yet)