I think you definitely want at least one healer in your party, and you probably want two when you can. Boldtusk, Kiril, and Rigard are probably the best four-star healers, as they have the defense to take a some strong shots and still get their specials off. Belith is probably the best three-star healer, but Hawkmoon and Friar are viable if that’s all you have. (If you have a five-star hero, obviously you use them, whichever five-star you’ve been lucky enough to draw, healer or no.)
You probably also want at least one or two debuffers on your team, if you can—Caedmon, Sonya, Sabina, and Melendor, I believe are the four-stars whose specials debuff. If you don’t have a debuffer, Boril and Cyprian can murder your squad. Once you have a debuffer, just keep their special locked and loaded so that you can dispel the “perfect riposte” when it goes off.
If you don’t have someone who can cure status ailments from your team, then Hu Tao and Colen can really be a pain if you’re not able to kill them quickly.
After that, it really is a bag you can mix and match from. The heroes I least like facing in raids and who I always target first when attacking are, in something like this order: Boldtusk, Rigard, Kiril, other healers, five-stars, Hu Tao and Colen if you don’t have an ailment-remover, Boril and Cyprian if you don’t have a debuffer, Li Xiu, Grimm, Tibertus, Gormek, and, if he is allowed to get off his special, Little John can be devastating.
Hope some of that helps, man! And good luck!