For each Interval/Score achievement you will need to maximize your points gain. This is easier without a partner as your individual score is needed to earn this achievement. Interval 06 feels constrained and introduces another new enemy; Alma's Monstrosities. You can earn 6 or so weapon challenges here (Strader, SMG, CAS, G3A3, Frag/Flash/Shock, Laser). There's not a lot to this level that hasn't been done before. You can continuously kill the summoned enemies from the Phase Caster and when you get the Enhanced Powered Armor be sure to work on its melee and 20 streak challenge. The only hard part would be when two helicopters attack you at once. Be sure to use the EPA's shield when fighting them so they don't force you to exit it.