Best advice I can give is to keep with it. It gives finishing those trouble characters a sense of accomplishment when it finally happens.
Hoard.Hoard your CP, LTs, and Taco Tokens. Just hoard for a while and work on the 4-stars you can level/champ. Let those annoying 5/1/1 characters develop a little organically from progression rewards and 3-star champ rewards. After you get a pile of Taco Tokens (100-200) keep an eye on those Taco vaults for 4-star covers you need and see if you can't get lucky. If you get a top tier 4-star to 12 covers, consider spending CP to buy that last 13 cover and be done with it - I did that a lot and don't regret it one bit. (In fact, I used CP to buy Peggy's 12th and 13th covers when I had her at 5/5/1.) If you have any spare HP, watch the Heroes for Hire sales for 13th covers as well - the game is crafty about putting ones in there you can use.