Strategy 1 (The one I like to use)Edit
Note: This strategy is for players who added at least 1 for each of the 5 skills.
First, before fighting a boss, have at least 10 potions that heal around 40-70% of your health. Also have smaller potions in case of emergency.
When you start the fight, use Absorption. Whether it works or not, click Glacier Strike next. During the charging round, use a potion if your health is low or use a special attack. Strike with the Glacier Strike.
If it doesn't work, do it again. Remember to watch your health. Keep repeating until the boss is down to less than 50% health, then attack with normal attack. You can use either a Life magic wand or an Ice magic wand.
The Nature Wand, a life magic wand created by Denithir, is highly recommended as it has the Spirit of Oak special ability, which heals around 20 health every few turns even when you attack, saving you one turn to consume a healing potion.
The Wall of Flame will cause 1 damage every turn if the boss manages to hit you, making it a must. Fireball or Snowball hit up to 5 damage if not resisted, so use them if all your regular attacks don't seem to be doing any damage.
If you don't add to all, and specialize in Shock/Spectral/Life (which is OK, I guess), you have to boost your Evasion, Lifesteal and your Shockwave (Other necessary skills are increased along with these) as much as you can. Once again check your potions. If you have too few potions with you, kill lower level monsters to obtain your potions and then sneak back to the boss as killing monsters near your level consumes too many potions. If you're playing a quick game, this also saves time in killing the monsters as lower level monsters have less health.
Start the fight. Again use Absorption (but this time with no Glacier Strike), then attack as per normal.
This strategy takes much longer, but hey, you chose not to add to Fire or Ice!