Why Tanks Are Necessary
Positioning is important. Proper placement could determine the outcome of the match. The idea is to out-damage the opponent by letting the monster in front take all the damage. If the tank falls, there's generally no defense left in the group structure.
Which Ones To Choose
There are many tanks to choose from. Sometimes, monsters with higher health are favorable. Other times, maybe reducing magic damage is the priority. These are just some of the tanks utilized in my matches:
| Fire | Water | Earth | Life | Death | Dragon | Neutral |
|---|---|---|---|---|---|---|
  |   |   |   |   |   |   |
Choosing a tank depends on the mode or ruleset of the match. For example, imagine there's a match with rulesets silencing both monsters and summoners:
Back to Basics 
Silenced Summoners
This would effectively render most melee-type monsters useless. In this scenario, range and magic are ideal because they can attack while behind the tank. The best tank to choose from would be Peaceful Giant as he has one of the highest healths in the game - 17 health!
Abilities to Look For
Shield 
Void 
Heal 
Stun
Both Shield and Void are useful abilities to halve the opponents damage. Shield halves melee and range damage (rounded up - except monsters with 1 damage deal 0 damage).
Heal is a must-have for those at lower leagues. It's a powerful tool against opponents who can't output much damage when monsters and summoners are low levels. A common combination used is Kron + Llama which is able to out-heal most scenarios. Heal restores up to 1/3 of the monster's max health (rounded up). Important to note: affliction and stun prevent monster's from healing.
Stun is underrated. It can be a game-changer. When a monster with stun hits, it has a 50% chance to stun and cause the target to skip one round. Most matches don't take too many rounds to finish. If the stunned target also has low speed, it could be a long time before they are able to get a hit in.
Abilities to Look Out For
Sneak 
Snipe
Sometimes it's easy to overlook the backline defense. Most people anticipate that their damage is enough to plow through anyone's defense faster than their backline is destroyed. Sneak can be a devastating ability. Silvershield Assassin is definitely one to look out for in this instance especially if paired with the Inspire ability which will allow 8+ damage per round.
Monsters with snipe strategically targets other monsters that are not positioned first (ranged, magic, or no attack). This ability could cause someone's defense to backfire. Another ability often used in conjunction is: Opportunity. This ability targets monsters with the lowest health.
Where to Position
Typically, tanks are positioned in front of everyone. In scenarios where the ruleset Super Sneak arises, it can be smart to add Enchanted Defender at the back of the group to bait melee-type monsters to hit it and get hit with the ability Thorns (cap of 2 damage). The best part is that this monster only requires 4 mana!
Super Sneak 
Explosive Weaponry
Explosive Weaponry is tricky to work with since it gives monsters the blast ability which deals adjacent additional damage equivalent to the main damage divided by 2 (rounded up). In this scenario, Gelatinous Cube would be helpful with its Heal ability if it is placed in the second position of the group to shield the rest of your monsters from the damage.
Reference battles: