Campaign 12-3 brutal.
Armiger, Madame Serris, Seeker, Magnarr and Orn are definetly worth lvl 60.
You have some uncommen Demons leveled as chickens, but not enough to upgrade all champs mentioned above. And don't use your epics as food, unless you are really sure you don't need them anymore. I just have to mention that, as you are at day 86, but I don't see High Khatun, the log-in reward for day 30.
For Spider, you need champs that can hold the turnmeter of the big spider down. Armiger is a great champ for that in general. Spider 18, with magic affinity, will be a roadblock, as he has weak affinity here.
Geomancer could be a key champ here, his hp-burn skill steals turnmeter at the same time. But it's on a cooldown, you need another champ to assist in tm reduction. I don't see any good option to replace Armiger, maybe you have to try it, even if he has the wrong affinity.
Team for stage 18 (one time use, 18 is not worth running, try to go on after you did it):
- Armiger for turnmeter reduction
- UDK to protect weak affinity Armiger
- Apo for healing and speed on your team
- Madame Serris for decrease def on the big Spider (everybody with that debuff could work, but as you want to lvl her up anyway)
- Geomancer to kill her
Strategy: Geomancer puts his hp-burn on the big Spider and, together with Armiger, keeps her turnmeter down for the fight. The small Spiderlings attack somebody, Geomancer reflects damage to the big Spider from his passive. Not as effective as the usual hp-burn on all Spiderlings strategy, but it should work.