Protector

The protector gains access to greatshield to help in their tanking. Like heavy armor, equipping a greatshield may be overkill on the defensive front. However, it comes with the benefit of letting your allies use you as cover, so greatshield + medium armor is better than normal shield + heavy armor if you have to choose between them.

With no getting past!, ignoring the protector's attacks of opportunities is basically impossible. It has sure hit and applies a slow at the minimum, so unless the target has high block there's little chance they can move at full speed away from a protector. The option to use a heavy attack exists as well. While a sure hit heavy attack is normally really strong, due to your heavy armor you probably do not have the RP to spare on it.

Provoke II is a fire and forget challenge ability that keeps an enemy taunted for the entire fight. You don't want to keep switching this around since the AP cost is too high for it, so just put it on the most dangerous enemy and leave it there.

Bullet Magnet is a tool for protecting your allies from ranged attacks. You have to use it on your turn, so you have to be aware of when enemy archers are in position to shoot your allies. It is a great ability to help win ranged battles, especially if you can find low or even high cover to hide behind.

The Secret Art: Shield Wall is effectively a party wide invulnerability against a single ability. The mana cost is very high, but this can save everyone's life when used against a powerful AoE attack.