If I solely focus on healing 300k (in full champ) is no longer the slightest issue for me assuming there is 300k to heal. Usually I try for the 75k and 25kills medals and sacrifice healing down to 200-250k or on a really good match I'll still hit 300k.
As of now, I have been consistently hitting 5 medals, and up to 8 medals if I am in a pre-made or we are just rolling the other team.
medals I can earn if I do well:
2.5k heal
5k heal (requires stacking buffs/adrenal/expertise spawn and I only recently started getting this one)
75k damage
300k healing
75k healing
10 kills
25 kills
killing blow
I can only earn a solo kill if, like the op said, I get lucky. I CAN get them, but it takes so long that it usually gets interrupted. I have tried for the 2.5k damage medal but it doesn't seem to be working out just yet.
Edit: I just wanted to add that as a healer for the longest time it was impossible for me to be a decent healer while earning over 4~ medals.