Amoongus and Slowro make an absolutely amazing defensive core. They cover each others weaknesses perfectly except for flying, dark and ghost. Amoongus runs specially defensive and Slowbro runs physical defense. Amoongus can take shadowballs all day and recover off damage from regenerator, giga drain and leftovers while slowbro can take Brave birds remarkably. THey both have regenerater so they can easily switch between each other absorbing resisted hits while the toxic that either one can easily set up chips away at a foes health. Amoongus can clear smog away boosts when an opponent tries to use their defensive nature as set up fodder. Unfortunatley Yanmega kinda screws them over but I normally use them in OU with Heatram to form an absolutely AMAZING fire, water, grass as well as a less common but still great poison, psychic and steel core
You can't use Regenerator Amoonguss in OU, since it hasn't been released in the Dream World yet. I'm guessing you meant in DW OU?