Well, Guardian is a beast.
However, each beast have it's weakness. In case of guardian it is his shield. It is not wide enough to be flanked-safe, even "extended shields" leaves large space to strike. Oh, yes, armor is good, but not-that-good to withstand double tach strike or reaper salvo. More, shield initial damage ratio is 1. That is a standard value, but it's closest player-available contender (paragon) has 0.6 and +250 more initial dissipation rate. And a closest non player-available contender (radiant) has 0.6 and +500 respectively. If not that incredible mobility and autoforge it would be not-that-fearsome.
P.S. It really has to be like 80-120 DP, not 40. I mean, it's 450 OP - be honest it worth it!