Here's some math behind it. Keep in mind I am not calculating hero base since all testing was done with a defending destroyer.
So I had my destroyer vs a guildies evo Orc.
Without a FG, and several runs, the min deflect dmg (active skill) was 3,145 and the max deflect dmg was 3,422 giving a ∆ of 197 and an AVG of 3,324.
if I take the 3,324 avg deflected dmg with a 7/10 skill (40% dmg deflection) and calculate the actual dmg, it’s 8,160 avg total dmg. If I was to add a 3/5 FG (14% dmg deflection), my calculated dmg deflection should be 48.4% which equates to 3,949 dmg deflected.
So with the FG as a crest and several runs, the min deflect was 3,200 and max deflect was 3,437 giving a ∆ of 237 and an AVG 3,299. If this was theoretically 48.4% dmg deflection, the avg total dmg would be 6,817.
This is a -∆1,343 from the runs with a non FG destroyer. Now on the runs where FG was a crest and the proc was NOT active, the avg dmg deflection was 1,158 with a ∆ of 212 between max and min. Since FG was a 14% dmg deflection, that would give an avg total dmg of 8,271 which is about a 1-2% difference between both tests.
So, in a nutshell, Flame Guard does NOT stack with Devastation. When the skill is NOT active, FG will work.
I have a climbing bubble...
My bubble beats your bubble…
Hear it burst… *boom*
I lost my bubble :(