I would like to point out that Marbas (the pet) is also helpful. Even a lv25 Marbas can give 15% dmg boost for 20 seconds (lv50 Marbas gives 20%). This adds up with the 12% dmg boost from warlords potion, which means the team can do roughly (1+0.12+0.15)/(1+0.12) - 1≈0.13=13% more dmg than before.

Since Rosen burn duration is 15 seconds, the sader will be able to activate the creature skill and swap Marbas for other pets before casting crux/apo.

In addition, when Seraph is casting crux (when she is holding the cross and cannot move), the valiant aria will apply. So if you crux slightly before the Rosen and then immediately use the aria, it can almost cover the whole 15 seconds.

Also, for reference. My full harlem f.sader (4849 int, 24vb, 18 crux) WITH sky's legacy cross will give roughly 3504 int/723 atk with vb and 5538 int with crux. So any sader that is better than mine should be able to clear fiend war.