The way I did it might not be the easiest but I think it's the fastest, considering that in order to use the Gunslinger and the Seal Artist Pet, as the other solution sugests, it would require you to gather at least 450,000 coins to be able to purchase them. My way required me to gather and spend only 118,000. But might require a bit more skill.
Before I start explaining it, I just would like to comment that a while back after I started this game I was saving as many coins as I could and so I was able to gather up to 700,000 coins right before every other aspect of my progress in this game got reset for no apparent reason. I was VERY frustrated at first, but that also made me even more determined to finish it. It is a long grind and as a general rule I hate grinds, but I like the other aspects of this game and I only play it on my free time, when I'm unable to do something more productive (such as when I'm riding the bus to work). Now I play more conservatively and won't stock up coins if I can use them right away. Won't take chances.
Back to the solution then, what I did after my progress got reset was to save up as much as I could, not spending any coins on anything (<<< and this is key!) EXCEPT 2 items: from the weapons menu, the Old Bronze Sword; from the accessories meny, the Gentleman Hat. Now, that's not mandatory, but these items will (specially the sword) make your life easier and the many runs ahead less boring and shorter. The hat and the sword cost 18k combined.
After I got those, I just kept playing the first levels, specially the third one, which is the one we have to finish for this achievement. while I was saving up the coins in the process, my character was also getting stronger since every time they level up their stats get a bit better (attack, defense, etc). Naturally, I was also getting better at the level, advancing further and further until I finally got to the boss for the first time. It will take time and practice but remember that you also have a goal to collect the coins, so it's not like the failings are for nothing.
Now, another important thing is HOW TO SPEND the skill points in your skill tree. My first 10 points were applied into HP Mastery, then the next 10 into Shield Aura and then into Light Weapon Mastery. I only put 1 point into "Second Chance" after I was ready to beat the 3rd level boss and that's important because if you delay it like I did you will never be tempted to spend money on a revive, which would draw back your savings. Truth be told, when I finally beated the 3rd level boss I didn't even need that revive and my HP bar stayed almost full during the whole fight. So, spend that 1 point in that Second Chance skill if you want. The fifth skill option available in the skill tree at this Novice point is the Treasure Magnet one. I never wasted a single point there. So yeah, when I beated the 3rd boss I had a lot of unspent points (14 to be precise) and I was a level 43 Novice Adventurer class.
Then, once I finally had gathered 100,000 coins, I went to buy a pet. With that much money you only have 2 options: the lazy cat or the magical penguin. I chose the magical bird (magical indeed, cuz penguins aren't supposed to fly!) for its healing ability which helped me every now and then in my tries to get to the "3rd boss" with a full HP bar. My penguin was level 11 or 12 when I finally beated that boss. It wasn't easy to get to him, but it didn't take way too many tries neither.
So, summarizing, here's what you have to do if you're gonna follow my path and advice:
1) Set up your mind to not spend any money except for these 3 things: buying the Old Bronze Sword (8,000 coins), the Gentleman Hat (10,000) and saving up to 100,000 coins for the Magical Penguin pet.
2) Play the levels, specially the 3rd one, to practice and collect coins. It's possible that you may reach the "3rd boss" before having bought the pet but it's unlikely that you're going to defeat him before reaching around level 40 of the Adventurer class.
3) Spend your skills in the way I mention in this solution. Do not evolve your monster! (obviously).
4) Once you purchase the pet, you're good to go. Keep practicing until you are able to reach the 3rd boss and defeat it. In my case my monster was past level 40 and my pet was around level 11-12. The fight wasn't hard. The trick part is to get to him without loosing too much HP, which happens quite often by falling into pits and getting gang banged by groups of enemies.
Don't give up if you want this achievement! You CAN do it!
If this solution helps you in any way, please leave it a thumbs up to let me know. Thanks!