"It's frustrating, but it's possible!" - Meself
If you are struggling with the "No Mercy" challenge (kill 100 monsters in 150 seconds) and in case it is the last challenge you need to complete, I'm here to tell you how. The idea wasn't mine but I think it deserves better than to stay as a comment among 200 hundred other comments from the previous solution.
For this, you will need the Gunslinger (fox faced ranged monster), fully evolved and equipped with:
- the Gold Pistol (100,000 coins);
- the Megabot Helm (55,000 coins);
- the Boosterpack (55,000 coins);
- the Speed Boots (55,000 coins).
Wearing all that together, beside making your char look like a Transformer, shall boost your attack in +187.
You will also need to purchase 3 "Specials" from the shop, whose effects will last for only 10 minutes (which is sufficient to unlock this achievement):
- Boost attack (5,000 coins);
- Pathfinder (6,500 coins);
- Smooth Road (6,500 coins).
Now, whenever you are ready to give it a go, select endless mode. You will need to look for a specific stage, which is the lava/volcano/red background one, and you need to observe if at the beggining of the level you have bears as the first enemies coming at you. This is important because in this specific setting people have noted that it's more likely that the game will generate the 100 enemies you need to kill in the 180 meters that separate you from the boss.
If the game puts you in any other stage or if you have chickens instead of bears at the beggining of the volcano level, select the icon on the top right side of the screen, quit to the main menu and start endless mode again. Remember you have 10 minutes under the effects of those specials. It's enough time, but you don't want to waste.
In my experience, towards the middle/end of my run was when the game was finally throwing lots and lots of enemies at me. When I was about to reach the boss the notification (not the achievement) popped into my screen. I failed to kill the boss because the only skill I brought with me was "Charge Attack" (which I eventually used, every now and then against the 100 enemies). I didn't leave any other skills available in the other 2 slots (like Slow Motion or Firework) so I wouldn't risk tapping on them by accident and spoiling my try. After being defeated by the boss there was that screen which accounted the coins I won from completing the "No Mercy" challenge and still no achievement. The achievement only popped for me when I returned to the main menu screen.
I think it's also relevant to share that I had the Seal Artist pet as companion, because it supposedly boosts the attack stats as well. If you want to do the same, that will cost you another 150,000 coins. I didn't buy any coins in this game using real money.
Regarding other major challenges, a few of them glitched in my favor, like "Pentakill". Even if it hadn't, I figured a way to kill any of the bosses with a strong Gunslinger, which I will share in the related Marathon and Sacred Crown achievements solutions.
EDIT: ForcefulOliver2 adds to this solution in the comments:
"When you buy the powers(pathfinder and smooth road, no need of boost attack in my opinion, didnt use it)when you make it to the boss if you didnt get the challenge "no mercy" , instead of killing yourself, quit and start a new stage instead, the timer will still remain at 10 minutes, so in a certain way you have "unlimited" pathfinder/smooth road.
Thats how i got my achievement, and because of it, instead of always reloading to get the volcano stage i tried in any stage starting with bears and i got it on the snowy stage, right on the last ennemy before the boss.
When the challenge popped i have let the rabbit boss kill me and on main menu got my achievement :)"
If any of this helps you in any way, please leave a thumbs up to let me know. Thanks!