As with nearly all of these Eternal Achievements - you can set up a deck to target this achievement and play Gauntlet mode. In Gauntlet mode - you can set up a scenario where you prevent the AI from doing anything meaningful by setting up a defensive wall of units that will not be killed if the AI units attack. This creates a stalemate that will let you continue to draw cards and set up the necessary requirements.
This specific achievement relates to having two of the above heroes fully unlocked. This means that you have 8 influence in that specific color. So set up your favorite 2 color deck and include the heroes. The safest way to go is to keep them in your hand until you have all 8 influence for both heroes and play them together, but that is only to protect them from spells your AI opponent can hit them with.
Achievement will unlock at the end of the battle.
NOTE: As of 08 JAN 2021 - there is a bug with Milestones (in game achievements) and Xbox achievements. The Milestones for In Bloom / Ascension / Brilliance are mixed up, so while the in game milestone pops up correctly - you will be awarded the following achievements Brilliance / In Bloom / Ascension.
TLDR: This achievement will pop when the Milestone "Brilliance" pops.