The easiest way would be to join a room with other people that will help you power level, but if you're doing it on your own just work through Deeper Well on Hard or Insane until you get to about level 30 and then jump up to the Endless Spires and run that to max level. Those levels are easiest to focus, especially with decent towers, so you can level well without too much effort.
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if you're 1-10 do Medium Deeper Well. If you're 10-35 do Hard/Insane deeper well. If you're 35-45 do Hard Ramparts 45-70 do Insane Ramparts. If you're 70+ Congratulations theres no way to get past this quick or easy.
If you want to get to 28 without wanting to cut yourself, just do the campaign.
Slower than leeching, probably even with Deeper Well spam to a point if you're efficient at build planning.
After that it's a grind. Pick any map you can reliably clear and try to clear it on a harder difficulty. Some maps are better than others for XP per clear, but spamming the same map over and over may not be your cup of tea. Survival mixed and pure strategy modes can also bump rewards up.
For your first playthrough, I'd recommend just playing through the campaign on medium or hard. My gf and I found it thoroughly enjoyable. Now that we have multi 74s in Nightmare, the whole thing is just a huge grindfest. Of course if you love grinding, then by all means, powerlevel your apprentice/monk/huntress to 74 and start grinding away.
Worker bees can leave. Even drones can fly away. The queen is their slave.