Battlefield 3

Easiest way to get this is to unlock the crossbow first, then as a support class (so you can refill your own grenades), equip your best gun. Choose a map you're familiar with and stay on the outskirts of a traffic heavy area (flag b on metro or damavand peak), throw down an ammo box, then fling grenades until you catch someone, switch to the crossbow and shoot the next guy in the chest or head. Finally, switch to your primary gun and finish it off.

Please do as everyone else said with the support classes. I just wanted to confirm this will unlock if you are revived. As long as you accept the revive your progress will not be reset.

While you´re playing on the Aftermath maps you can easily get the achievement.
Start playing with a support and make use of the grenade mod.
Play as usual with your primary weapon and try to get a kill with the crossbow.
When you got both, start playing very carefully and start spaming granades.

For further discription watch the video from "[P]ure [E]scalation [G]uides" below.

Noshahr Canals TDM is probably one of the easier places to get this, especially when you notice your team having a-lot of Assault teammates readily reviving you should you not finish getting your achievement, if you're downed.

-It's extremely easy to spam grenades into the center of the map where everyone floods through. Just make sure to have your frag specialization on and chuck them as soon as you spawn, toward the direction you notice a cluster of reds from.

-Getting a kill with your primary shouldn't be too big a problem here, especially if you are using a shotgun like the M1014 buck, SPAS slug/buck or 870 Slug. They're quick and it's close quarters. You only only need one kill afterall.

-The crossbow may be tricky, but if you manage to flank around to the back area where the small shack is that looks out to the crane, you're sure to find a player or two laying down with an LMG or people freshly spawning from the center to run down the ledge. Sometimes even someone simply standing trying to snipe. Try for the head-shot if you get the opportunity. If you have the HE bolt unlocked, use it like a shotgun, if you're gutsy, and play the center area.

Final note:
Playing a recon can work somewhat, if you don't want to play as a support.
-Make use of the TUG in the center, to keep track of players so you won't get killed from behind or surprised by an unexpected face-to-face encounter.
-The spawn beacon can be placed underneath the shack if you start playing there. Players will typically not even notice it, allowing for an easy spawn abuse if you manage to keep it up awhile.
The only downside to playing recon instead of support is not being able to sit on a supply crate for more frags or crossbow bolts should you not get a kill right away and have expended your supply of them.

Lastly, the video below is my own encounter of having gotten the achievement after just starting to play the Aftermath maps.

I got this in Scavenger mode.

I found it easiest to kill a guy with a grenade, then find either a crossbow or a regular gun and get a kill with that, then find the other and get a kill with that.

That way you don't have to unlock the crossbow yet.

The easiest way to do this is on Grand Bazaar Rush as the defenders - preferably on a server with a high ticket %.

First off, this technique requires you to have the explosive bolt unlocked for the Xbow and grab a support kit.

Now all you have to do is get into a position where you can use the Xbow to bring rubble down by shooting the sides of the buildings on areas of high traffic. A good place to do this is on the Right side (defender's) alleyway next to the B M-Com. You can shoot over the little dividing building to the sides of the buildings on the opposite side.

Once this is done (I would advise doing this bit first) you can then just start throwing grenades over the same building until you get another kill.

Then just sit and wait for somebody to inevitably run through so you can gun them down with your primary.

Can be simple to get, some times a bit off luck is needed.

Use a kit you are comfortable with but for the best chance use support and equip sqaud frag as your specliziation (or frag) gives you that extra chance.

Noshar Canals on team death match is possibly one of the easiest ones to get it from.

Simply get one of kill with the grenade, crossbow and primary weapon.

The crossbow does have a high damage which is useful but dont miss as its a pain to reload. You can be revived and it will not reset your stats.