1. It's sounds possible to just corner boss from all side and gun him down, but rarely true in webtoons(For plot reasons, +reality check.), most of people in low level are garbage and healers are garbage-proof class that is defined by A. Mana Pool B. Heal Intensity, both things have no relation to humans in korean low-apocalypses novels/webtoons but are part of both visible or invisible Sir System.
2. Lets quantify "skill", human starts with 10 skill, you need 15 to stab with spear into MONSTER, 20 to shoot an arrow into MONSTER, and 25 to either fence with sword or block with shield against 1 ARCHETYPE OF MONSTERS(Pattern of attack will vary greatly between humanoids, Insecticides and etc). Then there is magic with either 10 skill required or 15 depending if dmg depends on Skill Points invested or Mastery acquired, and soars into skies if it's "Learn to master." but in such case magic is probably rare and valued in which case 10 warriors are in team of 15 is staple and warriors are merely exchangeable meatshields. Now we have healers who need 10 skill in all stages to fulfill their job, they are either Skill Point based or Faith based which is never rocket science in korean internet literature.
Why would there be more warriors in team when on avarage it's 2 to 3 times harder to make a living + you are useless outside ?