Been paying close attention now over a few Guardian runs, and I can see that a single taunt skill such as Neutron Stomp will instantly direct the Guardian's aggro at you. Period. End of story. If you're losing aggro after that point, it's because another commando has used a taunt skill, or else you're not at the cliff edge and therefore the guardian is pushing the person he was aggroed against away, and therefore resetting his hate table. If you need to regen some health or otherwise recover during a Guardian fight, just don't use any taunt skills and he won't be interested in you.