To unlock 'Terror of the deep' you must score 11,000 points on the third campaign mission, South China Sea.
Firstly, difficulty makes no difference to your points tally as there is no multiplier, so its easier on easy difficulty.
*Headshots give a 25 point bonus
*Adrenaline kills give a 50 point bonus
*Multi-kills give a 100 point bonus
This is in addition to a normal kill (100 points) and a squad kill (50 points).
I also chose not to utlise the ENGAGE feature, as that racks up squad kills and halves your possible points for that section.
I would recomend using a steady AR/LMG, with your main gun being a sniper rifle.
Every kill you go for, aim for the head with a sniper rifle, and chain them together as fast as you can for bonuses.
If you see a group of enemies standing together, use the grenade. The multi-kill bonus is the biggest you will get.
If you are struggling, and you see a lone soldier, let him shoot you til your screen blurs. Then headshot him from close range, as you get a 75pt bonus (adrenaline + headshot).
If you reach a section where enemies appear and its a programmed route, restart checkpoint and utilise claymores for big points.
Dont be scared to restart the checkpoint if you don't do well.
Good luck !