How ironic.... a day after I confessed that I couldn't solo Gengar at level 39, the opportunity presented itself: Gengar raid with 8 minutes left. Suggested counters were 4 Tyranitar and 2 Groudon.... since the last attempt I had powered up 2 Tyranitar and, of course, caught me some Groudon. What made it easier: My team got a weather boost. What made it harder: My old, very laggy Galaxy S5.
First attempt: With minimal dodging, all 6 fainted.... and with my phone, rejoining is futile. But it was close - survivability would be key. I had barely enough time for one more attempt.
This time, I started out with Groudon, alternating with Tyranitar; swapped one Tyranitar for a 3rd Groudon with Solar Beam; and dodged Shadow Ball as much as possible. And it made all the difference - won with several seconds (and two counters) remaining.
This was a lot of fun - and 4 rare candy the icing on the cake.
P.S. I've done (only) 18 legendary raids so far, and those are a fun, social activity - but it doesn't compare to the feeling of accomplishment when you finally solo a tier 3.