As Mtld mentioned before, playing solo works best.
And knowing the game will do the trick as you know what to expect. Some parts are harder for some, I had little or no trouble in the diamond mine, but the nuclear submarine map really took some effort. Find out what works best for you and after this playthrough you will know the game.
As for stating the obvious: use cover, reload in time, etc. like every shooter.
A few pointers, which worked out for me:
Take your time,
use sniper and the masterkey shotgun frontmount, forget about machinegunning your way through. Use up all your grenades as supply crates provide new ones. Gas containers and other explosive items can signifficantly help you but can explode only once so timing is essential to make them as efficient as possible.
As for your partner,
the a.i. machinegunner, keep him close and have him watch your back as some maps/levels have multiple paths where a single npc can take you out from behind.
Inevitably you or your partner will get downed, stay close.
About the multiple routes,
make sure to clear them all before proceeding. Alot of enemy action is scripted, so they'll do pretty much the same every attempt. iow youll have to be in the right place at the right time and all clear when the next scripted event starts. Also backtracking and looking for another way in works well since the enemy's attention is still drawn to your first entry point.
as much as possible. For some parts I remember saving after every single kill just to keep going.
Final fight with the bmp,
I got lucky (glitch?) when the bmp got stuck behind the chopper and me and my partner could fire upon it from the far right side (bunker with supply crate).
By far one of the hardest achievements I've ever obtained, good luck! and remember, your cousin dances naked...