Just interested in some advice on how melee classes best fight against conjurers, necros, scions etc who may have a collection of supporting NPCs around them. I struggle to do much against them, especially if my character is relatively low in total hp. In fact, I would go so far as to say I am totally crap against them.

More specificity of what kinds of characters you're playing, what level ranges, etc. would help give some better advice.

But broadly, if player mobs are a problem for you, I can think of three basic strategies, only some of which will be applicable in any situation.

1) Separate the player from them or otherwise neutralize them. Dismissal, summon, drive, pull, locked doors, and more can be situationally useful. Stealth characters also have an edge here, since you can catch a necromancer prepping their army, a conjurer between servitors, and the like with more regularity. Woe is the conjurer who gets attacked between conjuring a demon and binding a demon.

2) Kill the mobs. Characters who deal area damage or damage to everyone attacking them, snare, making mobs who do area attacks fight you where they'll draw enemies in with their area attacks, etc.

3) Tank well enough and/or do enough damage that if you redirect to / keep focus on / use command denial to keep from rescuing and such, you can kill the conjurer/necromancer/druid/whatever before the mobs kill you.

There are a couple outlier strategies which don't fit neatly into any of these categories but I hope it gives you some ideas.

Edit: And as always, there's no faster way to figure out what the weak points in a combo are than to play it yourself and pay attention to what really cleans your clock.

Thanks for that. Lots more trial and hopefully not so much error!