Just making plans for 5*.
So, Shadow Lairs are the main way to get radiants now?
You can also do any Stratum 6 in the Arcade (not just Firestorm Citadel). Seek out arenas and danger rooms at depth 26 and below.
Try searching Google (not the built-in forum search) for something like "radiant site:forums.spiralknights.com". You will find many, many threads. And then focus on threads from the past six months or so (since the drop rate was increased).
FSC: Average of 21 rafiants per run. (highest personally: 36; lowest 6; average is 21.
FSC is the best consistent method.
However, any gate with a combination of a D27/28 arena and danger room, or 2 arenas, will average 27-30 rads per run.
But since most gates are filled with timewasters like decon zones, compounds, beast levels, etc. (Anything but clockwork tunnels or arenas), the chance of this happening is rare, and if a gate happens to pop up with good S6 depths, it is definitely worth the dive.
As an fyi, it costs 453 rads per 5* item to get to level 10.
AKA 20-30 vana runs. I recommend ignoring your shield, heating armor only to level 5, and then focusing on your weapons. It will save you about 453 radiants per shield, and 357 radiants per armor/helm that you make.
@Octoslash
I agree with octo but if you want to do an SL for rads go ahead ive done a few on elite and all had pretty big rad drop rates but its not worth the 1800CE you need to use.It would be cheaper to buy rads for 750 on depot rather than wasting 1800CE on a SL which would give like 30<. ( cant remember :P)
It differs from player to player. Experienced players tend to manipulate arenas as they are more than capable of clearing them in a timely manner. Less experienced players tend to rely on firestorm citadel as there are very standard strategies that work and the payout is fairly high, while merchants tend to just buy their fire crystals using the energy or crowns that they pick up selling stuff..