This is not a problem with the healer being incapable. This is a problem where one player allowed themselves to be surrounded rather than leaning on things like battlefield control from allies. Since this is apparently 2014 wotc 5e it's safe to assume that the player who allowed themself to be surrounded felt that ignoring support classes while putting themselves in that situation was a scenario with minimal risk when it should be one of extreme risk.
The spiders dropped from the ceiling and this kind of thing happens all the time.
Don't split the party in the middle of a fight. Don't split the party to seek out a second fight while part of the party is already in a fight either. I'm pretty sure that Sun Tsu wrote about the risks of battling on multiple fronts. This is not a problem of difficulty getting to someone, it's a problem of poor choices. If the party was split because of adventure design forcing it, there still is not a need because the purpose of that sort of fork is to deliberately make the party need to be careful and think about things a bit more
So they should have all stayed together so the drqagon could breathe on them all at once?
Also they had to disable 4 pillar which required them to be separate.
Finally have you ever played with draogn fear or ever played with the frightened condition. A frightened charactger can't move towards the source of fright and this means if you want to keep the party together NONE of them can advance on the enemy.
So I guess they should have just let the monsters attack them from range?Again. This is a bone headed choice to play action hero with assumed plot armor levels of safety rather than tackling the encounter with more caution and relying on support like battlefield control
They had to split the party to attack the bad guys in melee. It would have been stupid for the melee character to pull up short and just skip a turn and allow the guy at range to make ranged attacks at will or make a weaker ranged attack himseld
Self inflicted wounds caused by recklessly poor choices and a total absence of battlefield control seems to be a running theme
This was early in this particular combat and there is control in play, note one of the bad guys is restrained and the box on the north is an area that was turned rock to mud.
The PCs are arranged as they are to keep the Bull things off of the Gnome to the south.
How many games is the GM running here? Are there multiple GM's? The party should be working together and doing things like covering for each other father then whatever this chaos of main characters soloing at the same table was... See the running theme problem above .
These stills were from 4 separate games and 3 separate parties with the same professional DM who posts his games online, but pick ANY DM that posts their games online and watch the battles and you will find many, many, many examples of the same sort of situation where PCs are not all bunched together in movement range.
The videos I posted earlier were from this same DM's last 3 D&D games and when I posted the videos I also pointed out the one (out of six) battles in those games where the PCs could reach each other for most of the fight.
My original argument is that PCs would often be in position where they could not all get to each other. I don't know if that number is 25% of the time or 50% of the time or 75% of the time, but it is common and it is significant (and not meaningless). An analysis of every single combat from the last 3 D&D games posted by this particular professional DM shows that in about 25 of 40 rounds of combats the PCs can not all reach each other in combat. Any other real games you look at will show the same.