Just neg 800 ish AC should be enough that the /s 2 War can't jump in and tank what ever just killed the (probably) already superior tank.
Neg 1800 AC makes sure that there is very slim chance of surviving, what ever the cause of leaving /s 2 was.
Also remember how good +550 spell AC was on 4.3 robe? Imagine how bad neg 6000 spell AC is.
Even 1 or 2 trap mobs in EF almost killed Alpharious, despite I dropped /s 2 asap (a lioness and a skelly, perhaps there was 3 i forget; usually a total joke, but when they never miss and hit for max every time, well even something that mundane, becomes a real danger of man down).