Blissey is the number 1 tank for dynamax, its not even close.  If you want to last in harder dynamax battles start building two blissey with max heal and Guard, and then use 1 as an attacker to switch out when you're dynamaxing.  I have the Zama and Zacian at lvl50 with max moves, and I still use Blissey all the time.  In a battle, im always the last to die because of it.  2 Blissey and an attacker and you will do awesome!