Playing through the story campaign on easy you'll end up with about 10,000 aliens killed. If you are going for the Master Strategist achievement which requires 100 gold medals, you should get this achievement on the way. If you're looking for the quickest way I recommend playing through levels on Easy + Practice so you get extra starting resources. Below are the number of waves and aliens I got when chasing this. Level 02, 12, and 20 were the ones I focused on most.
00 - 10 waves, 89 aliens
01 - 20 waves, 387 aliens
02 - 20 waves, 403 aliens
03 - 25 waves, 498 aliens
04 - 20 waves, 294 aliens
05 - 30 waves, 486 aliens
06 - 30 waves, 409 aliens
07 - 30 waves, 444 aliens
08 - 30 waves, 404 aliens
09 - 30 waves, 371 aliens
10 - 30 waves, 407 aliens
11 - 25 waves, 476 aliens
12 - 30 waves, 620 aliens
13 - 30 waves, 395 aliens
14 - 25 waves, 419 aliens
15 - 30 waves, 491 aliens
16 - 30 waves, 560 aliens
17 - 30 waves, 497 aliens
18 - 30 waves, 521 aliens
19 - 35 waves, 606 aliens
20 - 35 waves, 785 aliens
Another good option is to play the Grinder game type on easy difficulty which consist of 100 waves of the easiest alien type, Walkers. You can beat these with a handful of fully upgraded concussion towers (6-10) and a strategically placed fully upgraded temporal tower.
12 - 100 waves, 1876 aliens
15 - 100 waves, 1876 aliens
17 - 100 waves, 1876 aliens