Level 20 Blissey

I'm wondering the same thing. Here is my impression, curious what others think. This is based of the mons I use against it, mostly 2500CP machamps.
- <2200CP, around level 25 for high IV: Blissey is not much of a defender, I use whatever attacker autoselect is giving me, to save time
- 2500CP/level 27: becomes OK
- 2800CP/level 32: good defender. A Blissey with ZH/DG and my 2500+CP Machamp usually faint around the same, depending on luck with lag and dodging
- 3100CP/level 38: I have to choose a good second attacker.

After gen3 and move rebalance, if nothing changes, I plan to bring all my Blisseys to >2800CP and a few to >3000CP.

I hate it that my 3200 CP Blissey is drained to a ratata after 2 hours. Niantic should change this. In general, Blissey become a pain in the ass above 2750 CP.

A lvl20 Blissey tends to be just under 2k. So it doesn't demotivated quickly. Higher level is always going to perform better but in situations where there isn't high traffic a level 20 one may be more desirable.

I usually use ttar for blisseys up to about 2700 b/c of potion efficiency, when blissey getting closer to 3000, I have to use machamp in threat of timeout.
But even 2000-2300 one is useable, it still take some time and decays slowly.

i don’t think players under level 30 can take down a fully charged 3000+ blissey on their own

Honestly.

It's a pointless question. Ever since the gym work around its not viable putting any Blissey/chansey in a gym so talking about their levels is even more wasteful.

What your better off doing is putting in an ariados in a gym it's more cost effective than a blissey.

after x amount of time the motivation whether its 700 cp or 500 cp left.. this will greatly disadvantage any pokemon and its level or what pokemon it is will not count and ultimately no pokemon will be a great defender after that many hours.

Generally the less a gym turns over, the lower CP mon you want to add. For example, I have a level 10 Blissey (CP 909) which is good for gyms that turn over in more than 24 hours. She will lose motivation very slowly.

A CP900 Chansey might be even better for such low-turnover gyms. It will have more HP, but a lower attack stat. But the gym might already have a Chansey in it.

Another trick is to add a high-level Blissey to a remote gym. After 24 hours, if she is still around, she will be totally unmotivated. Your friend then adds a CP1000 Chansey, and (maybe) feeds your Blissey a Golden Razz. Now you have a gym which will discourage attackers for a couple of days. Of course, it anyone really wants to take it down they can. It just takes time.