I Need help with Defender cl on too human, how to not die?

Alright, I know the defender cl is the MEGA AWESOME TANK Cl that shouldnt die, but hell, im lvl 20, and ive died about 23 times, mainly because of the Trolls, the missle ones, I do about 500-800 Damage, my Armor is 568 or more, Im middle tree, I havent leveled my Cybernetics very high, But my main tree, Defender Tree, I have my Armor skill up to 10, The first skill to 10, Spider to 4 or so, so how come i keep dying? Im good at killing and im good with Strategy, but what can i do? I Keep putting Reinforcement Runes on my Armor, I have a Reinforced Helmet of Reinforcement, odd name yeah i know, It has Prot +8, +8, +11, and I keep dying lol, any suggestions?