So, Thursday dungeon is the dragon flower dungeon for evolving pendras from 9k to 45k - the dragons you need for the fusion come from the "Hunt!" i.e random daily dungeon cycling correct? At 20 sta cost, how tough are they to complete?
Pendras take like 2.6k xp to get to max, then the dragon plant and 2 of the lowest level dragons from the daily jewel dungeon areas. The dragon plant can be found in tower of trials and tower of giants in addition to the thursday dungeon (green carbuncle, dragon seed, dragon plant, dragon flower, and dragon fruit all drop from the Thursday dungeon).
The jewel dragons are from the daily hunt ones. They're really easy to complete.
Typical battles are:
3x low level dragons
4x low level dragons
1x king dragons
3x low level + 1 high level dragons
1x king + 2 low level dragons
2x low level + 1 high level (almost ways the final boss encounter)
Low level dragons have a "good" defense, in that most attacks will hit for 1 dmg (I think the threshold is 6k attack to overcome).
High level dragons have a great defense, in that almost all attacks hit for 1 dmg (20k ripper attack hits for 5000)
King level dragons have amazing defense, and everything I've found hits for 1 dmg (ripper included).
Shiva's armor void works to nullfiy it, poison side steps it, post-orb-match dmg is reduced to 0.
I try to hold onto 4 of each of the low level for pendraga evolution (and 5 dragon plants). The kings are better xp, but neither are bad. I guess the new version in Japan has light and dark pendra dropping, so there's that also.
Hope that helps.