TL;DR: This is by far, one of the best gacha infused TD games available. Why would anyone go for the worse and inferior Chinese/English rip-offs simply because this game doesn't advertise itself as hard as they do?
Difficulty according to progress:
Early Game - Hard
Mid Game - Casual
Late Game - Extremely frustating (still doable if you are Kim Possible)
End Game - Jesus may cry
Mobile Game Tier. If you want to hit the spike fast, you probably will have to go ham early game and it takes an hour at the early stage. Once you hit the spike, you need less and less time to complete most stages till you hit the bottleneck. But even hitting that bottleneck, you don't need to spend too much time to play but just some time to do research and strategise. So round it off, about 30-45 mins per day.
Okay, the score I gave doesn't put this game high, but I will explain why I would consider it as one of the best.
I don't want to sound biased, so I will start with the bad, the ugly and the crap. The graphics don't look perfectly cool, the audios are barely better than the 8-bit games and it is a Tower Defence game. Not to mention, the gacha rate of getting a Black unit is lethally low, the amount of resources require to max out 1 single unit is extremely demanding. The recovery rate for energy is absurdly slow...
Before I get to the pretty, the cool and the sexy, let's get into the simple guide for a newbie. First, finish the tutorial and learn the basics. Then, look at your squad, focus on leveling up 1 melee unit (Gold Tier+), 1 ranged unit (physica/magicall; Gold Tier+) then 1 healer (ranged). Progress as much as you , and hold onto the Black Tickets you get at level 10 & 25. Get at least 150 Crystals for gachas. Draw 3 10x Gachas. Depending on the results, you can now pick your Black Units with the Black Tickets. You can pick the ones you want or pick ones that can adapt well into your squad.
Despite all the hard grinding and the average quality in both graphics and audios, the developer is rather fair in rewards and compensations. So even as a complete F2P, you can get really far by the 3rd month (I played completely blind, so it took me a little bit longer like a week or 2 more). I never expected 3 months of hardwork would actually pay off, but it did and I now can complete most of the difficult stages (except those that are truly God Tier, probably would need another 2 months or so to get there). The best part of this game, you can literally use 90% of all the Gold Tier or + units to complete the stages of the highest difficulty. Most of the possible combinations work as long as you know how to strategize them. And that's the fun. Most of the TD have so many limiting factors on the choices, this one, whatever the pull you have in the gachas, you can still make it to the top with event units and even the sub-par units. The stages are challenging yet not impossible. If you want a non-competitive TD that puts you up for a challenge now and then, with balanced units that allow you to come up with over thousands of feasible combinations, this is the one for you. OFC, if you are really stingy on the graphics and audios, this probably would not be the ideal game. Also, every new unit you get opens up gateways to more possibilities. Why not give it a try? BTW, just learn Japanese, I don't think there will be an English version ever again.