Go to low tiers (2 or 3), spam boss for 45-50 min (depending on your ability) then run around and open up chests for the remaining time.
You can farm a spark in <4 hours this way.
Went that route but it took me closer to 10 than 4.
Ooof..... got the 4 sparks now I need to farm gold....
This season seems to be somewhere around 10 hours solo again. Running Infernal Hordes lvl 1 to get up to T4, and then spamming IH lvl 2 (also a pretty decent way to grind cash..... somewhere around 5M / run).