Long Bomb states it requires players to "throw a frag grenade and hit five enemies without bouncing" to be unlocked, however this is no longer the case, you can use any veteran grenade to unlock it (though it still cant bounce)
but my method requires the Smoke Grenade
This is because smoke grenade actually has the shortest fuse of all the veteran grenades, we will exploit that in this guide.

You may want to equip perks or items that boost:

• Health
  
• Toughness
  
• Toughness Damage reduction

After you've done the previous steps you can jump in a match and follow along to actually unlock it. You can get knocked or killed with this method if you aren't careful so probably stick to Malice difficulty or lower (You can't unlock this penance in the Psykhanium btw)
Step 1: Approach a large hoard of enemies (It doesn't matter which types but preferably weaker ones)
Step 2: Purposefully have the enemies surround while you are holding your smoke grenade, than look straight up
Step 3: Underhand throw the grenade straight up while remaining crouched for the throw
If you are lucky the grenade will pop right before it bounces on anyone and unlocks the penance for you.
This may take a couple of tries (it took me a couple of tries using this method) but I find this way much easier than other methods.

Hope this guide helps, I found this penance a pain to unlock until I found out about this method.