Halo 3 ODST was such a fun game because the firefight so strategically inclined. Knowing little things could save your life. My group that I did the achievements with had created callouts for every map, and knowing them had saved our lives many times over. The jumps in the game were very satisfying. On Crater, I knew how to safely jump down from spawn or sniper rifle without any risk of fall damage. Some were extremely helpful. On Lost Platoon, I could jump from my Chopper farm (at sniper rifle) to the large tree, to the Warthog, way faster than taking the stairs. Discuss your personal strategies.
When I played through Firefight with my friends, we may not have had the best call-outs, but we moved as a group and would protect each other. It worked, and was quite enjoyable. Then there was Choppers on Lost Platoon, need I say more?