Burn and poison effect
 

Currently I am playing mage on impossible level. From lvl 8-15 I was only using a single green dragon to win battles (Lvl 3 order magic with Heal and Summon Phoenix- to burn them- did the job).
My question is: burn and poison effects seem to kill about 10% of the effected units per round (depends on their resistance) however the very same debuff did only 2.5-5% damage on the same enemy troop later in the game (Elon, Umkas). Has anyone experienced the same difference? Thanks for the answer.

Well, the burn/poison effect is based on a formula that you can find in the KBAP info thread. There is some randomness in it, so you probably just got a bad "dice roll", or the troop was bigger or smaller.

To save you some time, here's the formula:
Damage = Burn/Poison Factor * random number (0.05-0.1) * Current total troop health * Medal Bonus

Additional info on this subhect - here, Part X:
http://forum.1cpublishing.eu/showthread.php?t=10991

So yes, spells, and Phoenix's both will do a good burn/poison damage, as will large stacks. With full Medal Bonus (Which you will very likely get early on) you will do 6.5% to 13% stack damage per turn for three turns. This is a median of 9.75% damage by the way, and over three turns will do, on average 26% of the stack's health in damage.

That's a pretty good chunk!

Assuming you don't hit that stack in between, of course.