He's valuable early because he brings a pretty reliable ATK down. Most rare champs have their ATK down on a cooldown move, which means if it's ever resisted, you're stuck waiting another three turns before you can try again. Since his is on an A1, even if it is resisted one turn, you can still land it the next. The HP burn is nice as well, but that's more icing than anything else.

So, does he remain relevant? Not really. You'll get epic or legendary champs with pretty consistent ATK down and other tools as well.

That said, whether he's worth 6-starring or not depends entirely on the rest of your champs, and where you are in terms of progression. If the thing stopping you from getting top chest in Brutal, or potentially even Nightmare CB is the lack of ATK down, then sure, he'd be worth it. But otherwise, I'd probably focus on others - Geomancer for example will likely be far more useful.

On another note - you say it is very slow to rank people up. That makes me question how you are currently doing it? If ranking champs to 6* is your main priority, you should easily be able to get one a week, possibly even two. Are you farming Brutal 12-3 with a solo champ, and quickly? If not, that should probably be your number one priority.

Thank you. I've been k8nd of all over the place with my priorities, having just reached lvl 45, and gone through my 2nd clan war, I feel I'm starting to get a more narrow focus. I dropped trying to chase this fragment event to focus on a team to fight hydra. Which definitely seems to mean more lvl 60s. And I have 1 legendary and quite a few epics to work with, which is why I was reluctant on this champ that l seemed (and was,) far more important a couple weeks 

Don't take this the wrong way, but - you're probably many, *many* months away from Hydra being something relevant for you. Hydra is really intended for players who are at around Nightmare, probably even Ultra Nightmare Clan Boss, along with having all dungeons on farm mode, and a roster of around 30+ level 60s. At your stage I really doubt you'll be able to even get the lowest chest.

Your focus should be on farming food to rank up your existing champs that are worthwhile, and farming dungeons to get better gear for them.

I used Coffin Smasher all the way into my first UNM top chest team. He provided a backup source of attack down, his AoE damage reduction for the team shouldn't be ignored, and most importantly he's a nice source of HP burn. Accounts without Ninja can definitely use him through the mid game. And as an HP champ, you can build him incredibly tanky too, as as he gets more tanky his damage increases.

Other advice may very, but I think he's a top 3 rare in the game, after Apothecary and Coldheart. Him and Renegade fight for that third spot in my mind.

Coffinsmasher is excellent, if you are f2p definitely worth investing in. The caveat is, as kram said, you never know who you will pull... but like Quinn i used Coffinsmasher through unm clan boss.  He could even end up helping dungeon progression, albeit in slow runs :). Fairly certain I used on some quality dungeon 20 teams, 😆