I prefer the explosive grenade over the flash but in order to get the Jaw do I need to get kills with the Flash or can I continue using the explosive grenade? Im Rank 21 ATM

You need to reach whatever rank it is to unlock the Jaw, then you need to use a weapon unlock that you get by ranking up.

These are the ranks that give weapon unlocks.

7
9
13
15
17
19
21
24
27
29
31
33
37
40
43
45