BOMBERMAN ActZero

Getting anything above 30 is nearly impossible without boosting.

To get this, you have to kill up to 100 enemies in a row without dieing in Battle Royal, even if you kill yourself while the game ends in a draw. However, it doesn't count towards your count if your enemy dies to one of his own bombs or due to falling blocks.
But you don't have to do this in one sitting, as it saves your count as long as you don't die.

If you manage to kill 7 enemies with your bombs, you get 7 points in one game. But you have to be lucky to find even one gamer to play with.

My suggestion would be to boost it with as much friends as possible. If they already have some points, let the player with the most start to boost, obviously. Start a FPB Battle Royale (just in case you accidently stumble upon your own bombs). Tell your 7 buddys to stay still while the one who's boosting kills one by one. With that it will only be 15 games until one reaches 100 consecutive Kills. Rinse and Repeat.

I boosted it with only one friend, and it took ~3-6 hours for one player.

By doing this you also get a lot of Battle Score.

This achievement description is misleading. You do need to kill 100 people, but they have to be consecutive. If you die or kill yourself, then you must start your 100 kills over. The good news is you don't have to do this in one sitting. My advice would be to boost.