If you have the means to take up your Red Mags absolutely do that! He will shut down the Sentinel and Ultron bosses in Act 6.1 and his resistance/immunity could also be used for the biohazard cross bones. Sorcerer Supreme may also be a good option because she offers a cushion for any mistakes in that she can regenerate after Sp2. Obviously, Dr. Doom is a great power control champion to have, but may not offer the utility and resistances needed for some of the difficult 6.1 bosses.