Kinda a parallel to this question.
Setup
My players' party has the following composition of level 2 characters: Ranger, Cleric, Paladin, Barbarian, Warlock and Druid. Not all of them are present every session. In particular, the Barbarian was absent in the last two. If that's relevant, we're playing Lost Mine of Phandelver.
Problem
The Cleric and the Paladin usually are the front-liners. I usually split the attacks between both roughly equally, either round-based (i.e. one round the NPCs attack one character, another round the other character) or number-based (4 NPCs total, 2 attack each one). Even if I didn't, I don't feel like focus firing is unfair, as the party is constantly applying this tactic and is actually better than splitting fire randomly in almost every situation.
The Cleric is getting to 0 HP quite frequently, though, due to burst damage caused by all the NPCs attacking at the same time (as stated in the other question) or simply being unlucky. The Paladin got to low HPs (1~3) quite frequently as well, but not being dropped unconscious is clearly way less frustrating, so the Paladin is fine. The fact that the Cleric is getting unconscious while the Paladin doesn't, though, seems to be making the Cleric feel like I'm targetting only him.
I don't think they are playing (too) poorly, in terms of tactics – the cleric has a decently high AC (18) and HP (19 IIRC), so he can be the front line (although maybe he shouldn't, especially if he's getting frustrated by dropping down). The encounters are also not hard – even the ones I rebalanced for more PCs (as the adventure assumes 4 PCs and we have 5-6 sometimes) were, at worst, barely deadly and done while the party was at full resources. Other than that, these were normal/hard encounters.
Most of these encounters had melee enemies (e.g. Redbrands, which use sword attacks, bugbears and the Nothic – which was mostly using the double claw attack) and the party got to them in front-to-front combats, so actually moving through the front line, taking opportunity attacks, to reach the ranged backline would make even less sense than random targetting for me.
While "stop focusing him" seems a solution, it seems a really bad solution – both from the RP perspective of my NPCs (forcing them to be dumber than usual) and from a challenge perspective.
I have tried to explain to him that I'm not focusing him at all, even recording the attacks I had made and showing that in average I have targetted the Paladin more than him, but he said "Yeah, you attacked the Paladin after I was dropped to 0 HP" – ignoring that he got up and the Nothic kept attacking the Paladin for two more rounds after that.
I'm not sure I should simply play differently or try to talk to him, or to the whole party, or what. I don't want to sound rude saying "Welp, if you are going to complain about being focused, don't be the front line" or something along these lines, but currently I feel like this is the best solution – although I don't even know how to say it without him thinking it is even more personal, when it isn't.
Best Answer
The answer to both of your questions (this and the parallel one) is the same:
Stop giving all of the NPC attackers the same initiative.
That step right there eliminates the focus fire problem. That choice is the first element of a two part solution. You are the DM and you have this choice.
If you break up the attackers in to smaller sub groups, this burstiness is mitigated.
Now for the experience based part of the answer.
The DM in our first campaign in 5e taught me a valuable lesson; he broke the NPC enemies into groups once the number of enemies were larger than 3 or 4. He also had leaders, or different kinds of monsters, roll a separate initiative. All but one of my other DM's have done the same. You can watch the difference it makes at the table. A side effect of this is that the battle gets to look a little more like a tennis match: a couple of their guys attack, a couple of our guys attack, etc.
Example: seven goblins and two ogres fighting a party.
For a level 2 party, I'd spread it out because Low Level D&D 5e is swingy.
My advice on "the dice are fickle" applies to this also. This approach spreads initiative out so that party actions are interspersed with enemy actions. It's still swingy, but it is less bursty for one side also.
Second element of a solution: a primer on tactics for your players
Sit down with your players and discuss tactics. While each group has its own opinion on how close to "combat as war" they want to get, your choice of playing "smart monsters" (IMO a good one) means that the players have to learn
how to play smarter not harder.
Sometimes, it takes some DM coaching to get them working in that direction.
A tactical point to discuss with your players for this particular problem at 2d level: what are the spell casters doing to slow down the enemy? What crowd control or "dividing the enemy" measures are they taking?
You are the DM, so coach them. Almost every DM I've had over 4+ decades with this game has done a little, or even a lot, of coaching. It comes with the role.
As a final observation on your problem: if, even when faced with the facts, your player still thinks you are out to get him, there is only so much you can do as the DM. Some people are simply like that. My suggestions above may help mitigate that so that this player's perception changes.