After you finish Act 11 (Titans) the next act is the jungle that is inbetween the undead act(act 7) and molten act (act 8).
After that you can walk to the trees at the back of that jungle to get 2 chests and then help move the two villagers back to the spawn for another chest, then act 13 is the frost castle north of the mountain peak waypoint (act 6).

(Also in the jungle make sure you find the egg boss that is hidden to the right of the troll boss in the trees)

eh excuse me jack, I cannot seem to find the act 12 as there isnt a jungle between undead and molten act? I am s confused. Ive played this mode for quite a while and never figured out where is act 12 let alone act 13