The defense / offense balance you create when picking out your hand can be thought of as a spectrum with 100% Offense--------------------100% Defense being the two polar edges.
Ideally you'll set up a hand that is balanced with a coherent Offensive attack and a stout well rounded Defense, but there are certain scenarios where you may want to go heavy on the Offense or Defense based off of your opponent, the rule set, & the mana cap. Offensively there is the possibility of attacking 5 different positions; Tank (1st Position), Blast (Tank & 2nd Position), Snipe (1st non-melee card behind Tank), Sneak (Last Position), & finally Opportunity (opposing card with the lowest HP).
Unless you're psychic and know exactly how your opponent is going to attack you'll need to consider how you hand's defense will stack up vs each of these attacks. The next thing to consider is what variety of attack you'll be facing. Some cards will have very high defense against magic attacks while being vulnerable to melee or ranged attacks, while some cards will have strong physical attack defense but be vulnerable to magic attacks. Only a few cards have strong defense against both varieties of attack and they are some of the more expensive cards in the game, Lord Arianthus and the Chain Golem being the best two examples of a strong balanced defense at this point.
When putting together your defensive strategy the first thing you're limited by is the 7 card limit you have to put together a team. Is your summoner going to help offensively, or defensively? The next limitation you'll have is the mana cap. With the high or unlimited mana cap games you can be confident up to 3 and perhaps all 5 positions will be taking damage every round. Lower mana cap games are more difficult as you may only be able to use 2 or 3 cards after picking your summoner. This turns into a guessing game as to where you put your defense and what kind of defense you choose.
The next factor to take into account is your counter attack. Cards with abilities like Retaliate, Reflect, Thorns, & Return Fire will damage the attacking cards in some circumstances which make them very valuable defensively. Placing these cards in the right spot will sometimes be enough to take out a significant portion of your opponent's attack.
In order to get some sort of objective apples / apples idea of how the various Splinters and cards stack up defensively I use the following formula;
Physical Defense Rating = (Card HP+Armor / Card Mana Cost) and add +1 if the card has the Shield Ability.
Magic Defense Rating = (Card HP / Card Mana Cost) and add +1 if it has the Void Ability
With this in mind, here are some of the most used Defensive monster cards I have access to in my collection;
The Life deck has some strong defensive cards. With it's heavy armor and the Shield ability, my Silvershield Paladin has a physical defense rating of 3.2. While his magic defense is only 1.4 the Reflect ability gives it some appeal as an all around Tank defender in low-medium mana cap games. The Clay Golem has a mediocre 1.67 Physical defense but a very good 2.67 Magic defense, making it useful against opponents playing heavy magic attacks.
The Death splinter relies more on defensive buffs and special abilities than overwhelming base card stats. The Lord of Darkness has a stout Physical defense of 2.57 making it a good Tank defender against Melee & Ranged attacks, it's offensive power makes up for a paltry 1.29 Magic defense. The Animated Corpse is useful against magic attacks in low mana cap games or in Little League rule sets, with a strong rating of 2.5 against magic but only 1.5 against physical attacks.
The Water splinter holds some of the strongest Physical attack defenders I have in my collection. The Spineback Turtle is my best low mana cap tank with a 2.5 (PHYS) / 1.5 (Magic) defensive rating. Add on his Thorns ability and this is one of my favorite defensive cards in the game. The Frozen Soldier comes in with a high 2.83 physical defense rating making it useful in Melee only games or where your opponent is using heavy Melee and Ranged attacks.
The Fire splinter is more offensively focused, making me rely on my Neutral cards for Tank defense in most cases. The Fire Beetle is useful in the Snipe, Blast, & Rear guard spots with a 2 (Phys) / 1.67 (Magic) defensive rating and the Naga Fire Wizard is a premier Snipe or Rear Guard defender in higher mana cap games with its Thorns and Shield ability which help offset a very pedestrian 1.88 / .75 base defensive rating.
The Earth Splinter contains some strong defensive options, probably the most surprising would be the Failed Summoner. While his lack of offense makes it's usefulness very limited, a mana cost of 2 and high HP of 7 give him a very strong 3.5 defensive rating vs physical and magic attacks. Couple this with his Reflect ability and this card is quickly becoming one of my favorite defensive pieces to use. The Earth Elemental similarly has a strong defensive rating of 3.33 versus physical and magic attacks, making it a good choice for Snipe, Blast, or Rear Guard positions in your lineup. With the Healing ability on top of this it is usually strong enough to hold off an opponent using Snipe attacks long enough for my own attackers to get 2-3 rounds of strikes in.
The Manticore is the only card in my Dragon deck worthy of any defensive mention. A sturdy 2.0 rating vs physical and magic attacks coupled with the Thorns ability make this a useful card in my types of games and rule sets.
There are many cards in the Neutral splinter worthy of defensive mention. Lord Arianthus comes in with a strong 2.6 rating vs physical and magic attacks, add to this his Reflect & Thorns abilities and it's easy to see why he is one of the premier defensive cards in the game. The Chain Golem has much more to offer offensively, but his 2.5 (phys) / 2.0 (magic) defense make him one of the best tanks in the game in higher mana cap battles. The Peaceful Giant and his 2.6 rating vs physical and magic attacks make him a go to in Back to Basics rule sets. The Enchanted Defender is very vulnerable to magic attacks, but if placed in the Rear Guard positions his armor will give me a 2.0 physical defense. Most importantly, his 1 HP will draw the opponents Opportunity attacks and his Thorns ability will do several rounds of damage versus the opposing teams specialty attackers. Also worthy of mention as a Snipe or Blast positions defender, the Cyclops with a 2.17 physical defense thanks to the Shield ability is a very useful card to use if you are focusing on Ranged offensive attacks.
My opponent in this battle had been consistently playing hands with heavy magic attack and almost exclusively magic defense. Using a combination of the Clay Golem and Crystal Werewolf work together to provide my Tank position with strong magic defense while my rear guard is protected by the Crystal Werewolf's thorns. I take a chance and leave the middle of my defense vulnerable and it pays off in this battle.
Life vs Death
A battle with the Little League rule set means it's most likely my opponent will be using physical melee or ranged attacks. The Spineback Turtle anchors my team's tank position while the armor bonus from the Crustacean King and the -1 Ranged attack given by the Naga Windmaster & its Headwinds ability neutralize the opponents offensive attack and allow a flawless victory where my team ends the battle with full HP.
Water vs Fire