How to defeat Satan

 just attack with your strongest attack

get demons with resist fire
and 1 null fire for your support demon with invitation skill[resurect and swap skill]
i recommend your main character having Invitation skill too because your demons will most like die here just use invitation and keep spaming fang breaker[attack down skill] and your main damaging skill

resist fire only for ur main and 2 other demons, the reason for this is for the whole party to share the fire attacks therefor lesser chance of a party member dying

demons with null fire will not get attacked therefor your other teamates that doesnt have will get spammed all the time increasing the chance of death

AVOID making him smirk by  having your demons weakness exploited
if he smirks and land a almighty attack on you its the end

Skills needed

Fang breaker or other attack decreasing skills as long as you can maintain it
mediarahan - full heal
Imposing stance - backup skill if in a pinch
Invitation - to summon back fallen demons as soon as possible
Samarecarm - to resurect demons or your main character

Optional skills
Energy drain - to help your demon maintain their skills

Recommended partner
Navarre - mainly because of support skills


Recommendation
use high level demons with high hp[900+ preferred]  with resist fire
make sure his attack is lowered by spam fang breaker or your preferred skill for lowering attack
make sure most of your demons have samarecarm
do not use tetrakarm or makarakarn because satan will just use almighty attacks if you do that

Reminder
this guide is for War difficulty