This trophy seems to be buggy, so I've listed off the guns and equipment I've used in order to unlock this trophy.
Get a kill with:
1. SNIPER RIFLE (Any will do, I used the Mosin-Nagant Camo)
2. SMG (Sten Mk. II)
3. ASSAULT RIFLE (MP44 Camo)
4. PANZERSCHRECK
5. TRENCH GUN
6. PISTOL (Luger P08)
7. SILENCED PISTOL (Semi Auto HDM)
8. REVOLVER (M1917 Revolver)
9. M24 GRENADE
10. DYNAMITE (Throw at an enemy and shoot)
11. LANDMINE
12. TRIP MINE
13. S-MINE
14. KNIFE
If you're unlocked these weapons and are struggling to find a decent place to grind out the trophy, create a loadout with the weapons and equipment you think you need to progress with, start "Siege of Tobruk", kill the first 3 enemies and destroy the truck, make your way to the first dugout and save your game here. Rinse the two enemies, load last save, and repeat.