The following was posted on a previous forum by Star Troll and works still
If in any doubt about what to do consult the guild

Let me give you guys the formula for dispelling:

Main colour mana needed = 2 x (random factor) x (casterSL / dispellerSL) x min(1,(dispellerXPlevel / casterXPlevel)) x spellcost

This looks complicated, but I'll break it down for you (and give some examples).
Main colour mana needed: this is the amount of mana (in your main colour) you will need to dispell the enemy spell on you.
(random factor): this is a random (duh) number between 0.9 and 1.1. It's different everytime. To be on the safe side, I always use 1.1.
(casterSL / dispellerSL): this is the Spell Level of the caster (the enemy) of the spell, divided by the Spell Level of the dispeller (you). You can find your Spell Level in the [Status] screen and you can see the Spell Level of the caster on the [Enchantments] screen.
min(1,(dispellerXPlevel / casterXPlevel)): this looks tough, but it just means that if you are spelled by someone with a higher XP level than you, that you can use the difference in XP level to help you dispell. The maker of the game (Zeb) put this in to help newbies dispell spells cast on them by veterans.
spellcost: the cost of the mana to cast this spell (25.000 for Eternal Winter and Flood, 20.000 for Swamp Plague, 10.500 for Vegetational Decay and 4.000 for Frightening Howls).

Now for some examples:
Suppose Darkness has cast Eternal Winter on destroyer. Darkness has an XP level of 20, destroyer's XP level is 6. Darkness has a SL (Spell Level) of 420, destroyer's SL is only 60. Then:
Main colour mana needed = 2 x 1.1 x (420/60) x (6/20) x 25000 = 4.62 x 25000 = 115.500.
This means that destroyer can put in 115500 in the box next to 'Wild' and dispell the EW like that (same goes for Flood).

Another example:
The brave LPP mage ChaosRain [#1415] has cast Eternal Winter on the evil RPG mage ~±‚±~ [#139]. ChaosRain's XP level is 14 and ~±‚±~'s XP level is 13. ChaosRain's SL is 350 and ~±‚±~'s SL is 300. Then:
Main colour mana needed = 2 x 1.1 x (350/300) x (13/14) x 25.000 = 2.38333 x 25000 = 59.583.
So ~±‚±~ can put 59583 in the box next to the word 'Wild' and dispell the EW.


Please note that I write that this is the mana needed, when using your main colour. If for instance destroyer doesn't have 115500 Red (=Wild=Mountain) mana, he can use other mana too, but at an extra price. Using mana from one colour away (Swamp or Green in this case) will cost 1.1x as much (so 115500x1.1 = 127050), for the other mana colours (Blue and White) he will need 1.25x as much (115500x1.25 = 144375).

A further option was originaly written by Mulch of LPP. Important to know which spells to dispel 1st

If another mage has cast a hostile spell on you then a symbol like a purple flame will appear in the right hand pane of the game screen.
Do not use any turns while you are enchanted by hostile magic!
If you go to the enchantment screen (left hand side half way down), you can see what spells have been cast on you, including your own.

If you have enough mana you can dispell these enchantments, using the following dispell calculator:

http://student.stunet.se/vector/t5p/calculator.html

Your spell level can be found near the top on the status page, and the level of the hostile enchantment will be shown on the enchantment screen.
I still use the far coloumn on the calculator and round it up 2000 mana and this has worked everytime.
Be aware that mages can respell you very quicky after you have dispelled so take great care.
If you are spelled by multiple enchantments the order to dispell them in is:
1. Flood
2. Eternal Winter
3. Swamp Plague
4. Vegetation Decay
5. Frightening Howls

The reason for this is each spells ability to take forts, which DL touches on in his guide. This offers you the best chance of survival, and maintaining what forts you still have.

Read Mantes full guide on surviving BBQs at:
http://www.jaggu.org/forum/viewtopic.php?t=7847&highlight=dispell+order


If you do not have enough mana to dispell and have enough forts it is possible to counter and disrupt enchantments:
This works best with a reasonably fast internet link:
Quite often most, if not all the hostile spells on you are the work of one mage, so if you have some army left and attack the one mage spelling you, there is a chance to disrupt the enchantments.
EG "You lost 291 Land
Your spell Eternal Winter, was disrupted in the attack"
It is possible to hit off hostile enchantments, mana charge and rebuild barriers before people reallise what you are doing. What you need to do is open multiple windows and plan your actions carefully in advance.
Attack the mage enchanting you with as little army as you can send (you are only interested in dispelling and you don't want to hand out a free counter attack if you are in protection). If you have disrupted the enchantments , be very sure you have. Click on the next window, Normally set up for ten turns of mana charging, then click on the next window and rebuild your barriers to 2.5%.
At every step make sure you have not been spelled again
Hitting off enchantments is a risky method and feel free to ask for more help if you find you have to try it.

I was sat there reading the advice sections in this forum thinking they all look vaguely familiar, then I read this one

I suddenly remembered I was the mage called Mulch in LPP