Well, since there are no other solutions here for now, I'll share my two cents about how I've got this achievement.
First, I'll assume that if you want to unlock this achievement and even came here to read the solution it's because you care about achievements. And if you care about this achievement, you may as well care about the rest or at least most of the others from this game.
Being that true, my advice is to focus on this achievement only later on, AFTER you've bought the other classes (at least the Gunslinger and the Berserker). That's because, when you select the shop menu with each class, you will see many pieces of equipment for that class that cost the minimum of 3,000 coins. So, the cheapest and fastest way to unlock this achievement is to buy as many of those pieces until you hit the 30 required.
Also, have in mind that by doing this later on, the pieces that you really care about and need for other achievements will decrease the total you need to reach 30, so you will only have to buy a few more.
For the Gunslinger I've counted at least 3 weapons and 16 accessories (so, 19 items in total) that cost 3,000.
For the Adventurer and the Berserker classes, 3 weapons and 9 accessories each (12 + 12 = 24 items total).
I don't think that "Specials" bought counts towards this achievement, otherwise I should have unlocked it earlier.
If this solution helps you in any way, please leave a thumbs up to let me know. Thanks!