I figured I would add some tips for those of u looking to get this achievement. It may seem like a hell of a grind but i got it in one evening. Also, a side note for those of you wondering about the path to level 20, i have not played a single MP game yet (just did all training missions and got all to gold) and I'm a level 12.
EDIT: as far as getting to level 20 I got it while boosting for "Bling". It will come with time after getting all training missions to gold and then getting several player statistics to gold level so if you're looking to complete this game u shouldn't worry about having to make hours of sessions just for that! Thought that would be some good news for those of you yet to attain either of these achievements :)
Note: Gametype specific tips will be notated in [brackets]
Tip #1 - Head-On, Head Strong
Every vehicle in the game has weaker rear armor so use this to your advantage. ALWAYS try to face the enemy head-on and maneuver behind your enemies to get quick kills. Don't forget to WATCH YOUR BACK because your vehicle is no exception to this rule. Most enemies will come right at you but some will simply circle the map. Watch these annoying bastards' patterns and flank for quick efficient frags. I found alot of times these guys were the difference between gold and silver. Taking down a enemy as fast as possible will come in handy at all times but especially in annihilation type missions which focus on speed kills.
Tip #2 - Two are ALWAYS Better Than One
Getting multi-kills isn't just fun, there are great rewards for killing more than one enemy at once. In Annihilation mode (must kill the enemies as quick as possible) for each additional enemy you kill at the same time you are given 5 seconds back on the game timer. For instance, get a double kill and get 5 secs subtracted from the timer; get a triple kill and u get 10 secs subtracted, and so on. In Survival mode (survive as long as u can) it ADDS seconds to the timer to help u get to your goal faster. It may not seem like it but it is worth taking 4 or 5 seconds extra to setup that epic quad kill and get those precious bonus seconds that can make the difference in getting gold or silver. It is also worthy of note that u can use different things to get more than one kill at once (detonate that mine to kill one while u finish off another with your vehicles weapons etc.)
Tip #3 - Being Environmentally Friendly
In ANY mode be aware of your surroundings for they can be your worst enemy. There are explosive barrels pipelines and objects EVERYWHERE which can kill you instantly no matter how much health you have. In survival mode it may be best to simply blast the map to kingdom come just to make sure you don't accidentally shoot a explosive box right next to you and die .15 sec before u hit gold. The environment can also be your best friend, and with a bit of strategy, you can lure multiple enemies around a corner right into a flaming deathtrap. Also remember when u kill an enemy they explode, just like everything else, so: firstly, don't approach like u would in a shooter and run through your enemies as they die as you will lose precious health points or possibly die altogether; secondly, use this to your advantage and watch for enemies clumped together for easy multi-kills because if one dies, they ALL die! MWAAHAHAHAHA...uh yeah anyways on to the next tip...
Tip #4 - Camping is not prohibited
In Survival modes find a choke point where u can camp effectively to survive the longest. Spawns are random BUT there are only a certain number of spawn points per map. Learn, exploit and win. For instance, on level 8, "The Briar Patch", I have found its easiest to go to the bottom left corner of the map, hide behind the railing and let the enemies come up to me. With only two ways to cover, (directly below u and those that will come across the bridge) and no random spawn point right next to you this makes for the perfect spot to roast some marshmallows for a good 4 minutes or so.
Tip #5 - Secondary Weapons, Primary Goal
Secondary weapons make your missions 103 x's easier (yes, i mean 103). Placing strategic land mines/remote charges can even further increase the ease of camping a survival map. Using a singularity bomb to get that quad kill in annihilation mode will not only net u...
...but it will also grant a whopping 15 secs back on ur game time. Totally worth it. Use caution, however, and don't be too hasty to detonate that remote charge or stray too close to a engulfing S bomb to avoid embarrassing and time consuming suicides; because in this game, time is EVERYTHING.
Tip #6 - Practice Makes Perfect
There will be some missions you'll only have to play once or twice for that shiny gold star...then there will be the one you play for 2 hrs over and over again to finally get gold. For me the mission that took the most dedication of time was level 13. For you it may be different as we all have our own playstyles. Decide which fire mode you like (I found trigger fire to fit my style, if desired change with back button), "stick" (lol stick fire is the other mode. I made a funny hahaha...) with it, and these will all come in time.
Tip #7 - The Odd Ones Out
[Speed Trial/Shooting range]
There really aren't any specific tips I can give for Shooting Range/Speed Trial gametypes as there aren't any blaring shortcuts or quick completes. they are just straightforward, reach objective/destroy objective type missions. Although I personally found these to be the easiest gametypes as each time u play a mission your time should slowly, sometimes greatly increase, they deserve at least an honorable mention. The only real help I can offer is that I recommend completing the track every time u play it (even if you know your time is not faster) or at least until u have the track memorized, just for the sake of seeing the progression of flags/mines again. Every time you run through a track the fresher it will be in your mind.
I hope you found this solution helpful and don't forget to thumb up! If you feel something needs to be added or removed please comment or pm me and let me know how to make this solution more informative and applicable to the TA community before u vote negatively. Good luck!