I. The Defender Catch-22
To reach this goal, all defenders use a powerful tactic: boost up their own defenses and die-hardness, and then put enemies before a choice. Either attack the defender, or be punished. The key ability of every defender class to work this punishment is their mark.
Again the choices for the foe:
A:
Risk a miss against the defender
Concentrate your attacks on the defender. They have high defenses, high hit points and ways to remove or ignore status effects. This makes the enemy's attack likely to have little impact.
B:
Soak the punishment
Try to circumvent the defender and attack their softer allies. First of all, the defender imposes a penalty to this attack with their mark. Then, each defender class has individual ways to keep the enemy from reaching the target, staying near the enemy, reducing the attack's effect or punishing the enemy.
A good defender needs to make both A and B painful for the enemy.
If they neglect their defense, they are a liability to the party because it is easier for enemies to render them ineffective with weaken, stun and other effects. They also needs to be healed more often, which drains party resources.
If they neglects B, they'll be like a stone pillar that enemies can't push over, but they can just walk around them and kill their allies first, then finish them off last.
II. Understand your Defenses
There are 4 defenses to worry about: AC, Fortitude, Reflex and Will. The last three are also called NADs (non-AC defenses). For more information on how to keep your defenses high, check the build handbook for your class on the Character Optimization forum.
AC
Most of the monster attacks you'll have to shake off will be melee attacks. If you check the Monster Manual, you'll see that a very high percentage of melee attacks will be against AC. This also includes opportunity attacks. AC is your highest priority.
The big decision for all defenders (except the swordmage) is: Shield or not? The +2 AC of a heavy shield means that 3 out of 10, rather than 4 out of 10 attacks will hit you. On the other hand, you could wield a reach weapon for better battlefield control, especially as a fighter or warden, or a two-handed weapon for more damage, or two weapons for more attacks (tempest fighter). Each alternative leads to different tactics.
Shield wearers / swordmages should try to achieve an AC of 18 + level. Without a shield, a typical AC is 16 + level. It is often easy to boost those numbers by 1.
NADs
Monster attacks that target NADs usually have an attack bonus that is about 2 points lower than attacks against AC. NADs 2 points lower than your AC should suffice then, but the bad news is that it is difficult to get all of your NADs that high. Monster attacks increase by +1 for each monster level, so a 31st level monster will have a +30 higher attack bonus than a 1st level one. Your NADs rise only every second level, so there is a gap of 15 points that you have to fill.
Remember that you can boost two of your ability scores at level 4 and 8 of each tier and all of them at 11 and 21. But even if you raise two abilities that boost different defenses, there will always be one defense that you don't boost. At 30th level, two of your NADs will get a +5 bonus that way, but one only gets +1. You need to fill the remaining 10 / 14 points with feats, your neck slot item, and handpicked other items.
Fortitude
Fortitude attacks often do more than just hit point damage, they weaken, slow, push, poison, petrify and do other nasty effects that can hamper you. For Fighters, Wardens and Strength paladins, their main attack stat will raise this defense, but it's still a good idea to boost it further.
Reflex
Dragon breath and other area effects often target Reflex. It's the least likely to carry nasty side effects, but don't rely on that. Swordmages will have a good Reflex thanks to their Intelligence, and others can carry a shield to boost it.
Will
Monster Manual statistics say that this defense is the least likely to be targeted. But Will attacks are also the one that is most likely to have very nasty effects like daze, stun or even dominate. A party with a dominated defender who starts to hack away at their vulnerable allies has pretty much lost the battle. Don't let that happen to you.
Charisma Paladins will have the best Will defense. Fighters and Swordmages really need to watch out here. Consider
Superior Will and similar feats.
Secondary Defenses
What I call "Secondary Defenses" is everything that helps your character reduce the effect of a being hit. Since you're the designated punchbag of the party, you need to make sure that you survive being hit better than your fellow party members. There are a wide selection of class features, powers, feats and items that provide these.
- (more) hit points
- temporary hit points
- healing surges / higher healing surge value
- self-healing
- bonuses to saving throws
- free save rolls (Warden, Paladin's Virtue's Touch, martial feats)
- resistance against elemental damage or all damage
- immediate actions that provide any of the above, either reducing damage or negating the hit
Debuffing
Another way to defend yourself, and your party members, is not to raise your own defenses but to hand out penalties to monster attacks. Giving a monster a -2 penalty to attacks is the same as raising your own defenses by 2. Conditions you cause with your powers are also powerful ways to hamper enemies, including weaken, daze, stun, blindness etc. See "IV. Reducing Enemy Effectiveness" for more techniques of "Offense as Defense".