As an avid FPS and MMORPG player I must say that in my opinion, major and ultra minotaurs shileds regenerate WAY too fast. I have a level 13 Voidwalker, and I'm undertaking the first mission on Venus when we encounter multiple waves of Vex at the Conflux including one or two waves with 2 yellow Axis minotaurs and it is extremely difficult. I have increased the difficulty to 14 for this mission, however I've been increasing the level on missions since level 10. I sincerely hope this gets addressed.
That mission was a major stumbling block for me, Voidwalkers represent. However, the next one was far, far worse. "The Ishtar Collective"... lets just say that I dealt with nearly a dozen Axis Minotaurs on the final bit of defending there. Along with the Axis Hobgoblins. That's the single hardest mission in the game in my experience.