New DM here...

Then try something new?

Spells that use saving throws

I'm not wanting to get the player hit, I'm wanting not all bad guys to be morons and only do one thing to attack the heavily armoured guy.
Like what? Grappling? Aid another strikes? Flanking? Luring him into a pit trap then poking him to death with spears? Tripping him prone into a water filled hole?

I don't know what would work! Or if my players would be upset by some of this stuff!

A bandit with +4 to hit is gonna hit a well armored PC about 1/4th of the time. Just throw a lot of bad guys and roll a bunch of dice and the PCs will get hit at some point. Also, having a character with high AC is not bad. The player built his character to be hard to hit, so don't punish him for his choice by cheating.

First post worst post

im a little 'spergy with some aspects of prep/notes (though i blame how spastic my usual group can be). I use a LOT of note cards that i record different stat blocks and variations and such with. lets me keep stocked with encounters of a good range without having to dig though books for somthing that fits where they are right then and there. If i have to ad-lib i can grab one of those, and after being a bit inspired by Kingdom Death i have a few stacks of "behavior" cards that depends on intelligence level of what they are fighting. These let me keep off-the-cuff encounters looking a bit more planned instead of panicking to populate it and figure out how the npc's would act (for instance, for low intelligence i have one that says "beastmaster" so ill have somone with an animal or animals fitting for them. or one that says "trapper" and the main group will try to bait the party into a simple trap on the field. meanwhile higher intelligence ill have "spec ops" where there will be some sort of caster and a bunch of ranged users utilizing cover focusing their attacks smartly) not a perfect system, but keeps pace in the games that go a bit outside of anything i prepped. other than that, biggest time-sink in session that i ALWAYS try to have ready are small lists of appropriate treasure ready or what can be for sale in the market(s). i just pull a page and am ready to go instead of doing all the rolls there (adding or subtracting as warranted).

I've been dming 5e since the beginning, and 4e before that. 5e sucks but it is impossible to find a game with superior systems.

My notes are mostly setting notes. I have a hex grid in OneNote with hyperlinks to pages on every hex. Encounters are made up on the spot using the hex notes for content clues.

>Players attempt to hit enemy
>Enemy has high AC
>Players try different tactics; combat manoeuvres, spells that don't target AC, flanking, buffing attack rolls

Play enemies in the same way. If the enemies have no way of overcoming the PC's defences then it's not a challenging encounter, so if you want a challenging encounter give them the ability to overcome. Just remember that just because you know something doesn't mean an enemy will or should know it until it actually becomes obvious (they try to attack and it just bounces off), and even when designing encounters (or just altering preset encounters) to challenge a PC's defence make sure it's still ultimately up to the dice and that what you're doing makes sense and isn't just blatant fudging (e.g. the bandit wizard utters an incantation and the fighter flexes his muscles and smiles, alluding to a buff; when the fighter attacks he seems more focused and his strikes are more measured, alluding to him not using power attack this time); at the end of it they should be thinking, "Thank god I invested in that AC," rather than, "Well all that AC was a complete waste of time."

What situation calls for fudging?

DM asspain because the players are having hot dice?

Practically nothing calls for fudging. Accept it and move on.