Play Mission 15: Guardians on the Heroic difficulty. Once you reach the energy cores (large blue balls with moving rings around them) part way through the mission, clear out the enemies, and work your way up to the left side to find another energy core at the top of the hill. Once ready, climb the tall rock nearby and Ground Pound (jump, then hold [Melee]) the energy core to get the "Icy Cool" achievement. If done correctly, it should be destroyed with a single hit. In the same area where you get the "Icy Cool" achievement, you can get the "Double Stuff" achievement, which requires you to destroy two energy cores within a five second window. Damage the first energy core you encounter and the next one up the hill to the left of that one so they turn red. Make sure to also kill all of the nearby enemies. Then, split your team into two groups, and coordinate the energy cores to be destroyed at the same time. If you are in co-op, just time it with your partner(s) (you can also attempt to get both achievements at the same time). If you are doing it solo, command the AI to destroy the core and immediately destroy your core as the other one is destroyed. If you fail, reload the checkpoint.