At this stage in your game you need to work on all the colors, so I’d be “rainbow leveling.” Level up green heroes with green feeders (i.e. match colors - use your 1* or 2* greens to level up your Belith) and so on for the other colors. You do get a 20% experience bonus using like color feeders in that way, and there’s a better chance to increase your special skill as well.
Focus on a rainbow team to start with - one hero from each color. You’re leveling all over the place, and you need to focus down. My suggestion would be: Belith, Melia, Tyrum, Gormek, and Graymane.
I’m sure it’s tempting to level up Lianna, but trust me that it takes way too much food and feeders to level her up at the stage you’re in. The 4*s (gormek, Wilbur, Colen, HuTao) are also going to take a long time, but gormek is going to be a great help with his defense-lowering special skill, so spend the time now to invest in him. If you get Valen, Gato, Ulmer, or Gunnar, I’d stop working on Graymane and change to one of those.
I prefer the “one level at a time” style of feeding. It’s a compromise between the “feed one at a time” and “feed 10 at a time” schools of thought. Basically, use enough feeders (1* and 2*) to increase your heroes level by one, and then get as close as you can to the next level without going up a second level.
p.s. Please please please don’t level up Oberon. He’s just awful. You can never have too many dispellers.