Zombie Army Trilogy

Start a (Solo) Game with the Chapter "Labyrinth of Death" from Episode 1 and set the Enemy Spawn to "For 4 Players". Choose 2 Dynamite and 2 Stiehlhandgranate as next. In the Chapter, go forward and put the Dynamite on the Ground, activate all enemies and gather all at the Dynamite Place. Stick all Zombies together, throw 2 Grenades in the pulk of Zombies. You will earn all 3 explosion Achievements at once with this tactics.

An extremely easy way to do this (as well as being convenient and after witnessing a friend get a 25+ combo) is to start the second mission, Cathedral of Resurrection, and play all the way until the battle with the Occult General, keeping at least one stick of dynamite with you.

Once there, kill any zombies and shoot the glass skulls that fly around the general until he summons enemies for a third time. Throw the dynamite in front of the stairs at the altar, line up your shot, and pull the trigger.

A trick to make this a tad easier is to make it to the third wave and restart checkpoint, which will start you with no enemies but on the same wave, thus giving you a little bit of time to throw the dynamite and aim at it.

EASY SOLUTION!

Choose the first campaign and the first level,
play until you get to a large place with a lot of barriers in it.
I can't describe it really because everything looks the same lol,
the place is right after a checkpoint/safe house and the place is filled with red blinking stuff to shoot at.On the opposite street there is a bombed out cafe with ammo in it.
The left side and the upper right way is blocked until you survive the 2 main hordes of skeletons and some soliders.
After you exit the safe house get to the middle of the place and put down 2x dynamite in the middle of the place. and then just wait for the skeleton waves, train them around the dynamite, run away and throw a grenade and you will unlock all 3 achievements.

This trophy is very easy, you just need to find a large group of zombies and just launch a grenade in the group. The video below will show you an example to unlock it.