It's not hard!

In the game, see in "Team" the total ATK from you monster (TotATK).

First, choose in your friend list a team without any monsters with elemental advantage (if your fire, don't fight nature team) or monsters who will reduce your ATK (like Obelisis, etc.).

Then, if it's a leader skill, put her as lead without any subs (to have no team bonus/pet bonus, leaders pet don't affect the team). Fight this team and use your skill (becarefull to not have any buff/debuff), the damage over the time you will get will allow you to calcule the DoT!

100 x DmgOverTime / TotATK = DoT%

If it's the support skill, put only her as sub (same reason). This time it's harder because of the team bonus (and the pet bonus if you have one). If the team bonus is x% ATK, you have to get the ATKduringFight with the formula : TotATK x (1+x). If you have a pet with y% ATK bonus, you have to do TotATK + 10*y. If you have these 2 bonus at the same time, you should use ('TotATK + 10*y) 'x (1+x). When you have the ATK during the fight, get the damage from poison in the fight, and use the formula 100 x DmgOverTime / ATKduringFight = DoT%

Ho, I'm late!

Its never too late :) Yesterday I used her against another friend monster and figured it out. Still it's a good info. I didn't knew about calculating attack buffs included monster so next time it will be handy. Thanks Poulinou.