General reccomentations: 

Play with headphones; not only is it more immersive, but you will be able to stay alert as well as identify just what exactly is heading your way. If your computer can handle shaders, the Insanity Shaders certainly add an extra eerie vibe; if not, there are some lightweight shader options as well.

Unless you like a challenge, playing with keep inventory is reccomended. The modpack is quite brutal in terms of how much damage you will be taking. There are also some really beefy mobs and not many good ways to counteract them early game. Don't underestimate the little guys, either.

The more you explore, the more structures and loot you'll find, increasing your chance for survival. But, don't worry, you won't get lost; the helpful xaero's minimap allows for easy marking where you have and haven't been.

(CHECK WARNINGS BEFORE PLAYING)

Day Experience:

Mostly normal, with the exception of some smaller hostile mobs, which can become a real nuisance if not dealt with quickly. Just try to keep moving; the more you do, the longer you live. Try to stick to forests, since fields and open areas are more likely to spawn something.... big.

Cave Experience:

Pretty difficult; there are a lot of aggressive mobs that will spawn. Make sure to light up your caves thoroughly, and ALWAYS keep a bed nearby. Prioritize having a bed near wherever you're playing instead of where your main spawn is. If you decide to play hardcore, best of luck to you.

Night Experience:

Best stay inside unless absolutely neccessary. This is when everything in the caves take to the surface; if sleeping isn't an option, you should either have a good plan or great armor. And, a special spooky someone might pay you a visit. Good luck.

Server Advice and General Warnings:

- Set server settings to Allow Flying; sometimes, if you just regularly fall far enough, you might get kicked for no reason. Funnily enough, all of the included flying methods are completely unbothered by this setting regardless, and work just fine.

- Turn off mob griefing; with the Mutant Creeper in here, you do NOT want to be cleaning up those creeper holes. Keep in mind, though, villager breeding doesn't work with mob griefing off. Toggle the setting as necessary.

- Since this modpack includes Inventory Pets, I should let you know that there's one called the Illuminati Pet, which has the ability to spawn in ANY existing item, even the creative-only items. If you'd rather your players not have the ability to be literally immortal (even to /kill, shockingly) I reccoment you disallow them from having one.

- Overall, this modpack works well for a multiplayer server, where you can help eachother stay alive; and, bond over fighting against the nuisances. But, I have to tell you, there are a couple things that can cause problems there. I'd rather you know:

 1) The supplementaries hatstand should be avoided in a server. I've done some troubleshooting, but it sends a broken packet and the server is unlogin-able to anyone. The fix was to open the world save in a singleplayer world with the supplementaries mod removed, and then porting that new, hatstand-less world file back into the server.  If there's a way to ban a specfic item from existing, please do this. (In singleplayer, you may momentarily be unable to break blocks as a result of placing the hatstand. try placing any other block, and then trying again. I should also note that the hatstand is invisible and useless, so it's just completely not worth it in the end.)

2) Don't try to stack curios effects (trinkets and baubles), especially the arcing orb. It's meant to give you an additional 25% speed, but if you try for 50%.... well, the server'll go down. The GOOD news, is that other players still can log in, JUST the person with the double baubles cannot; have an OP-ped player put down a command block with the command { /curios clear (person) (the slot that either duplicate bauble is in) } and set the command to Repeat+Unconditional+Always Active. Then, you just need the double-baubled person to log back in. (DO NOT DO THIS IN SINGLEPLAYER, YOU WILL NEVER BE ABLE TO LOG BACK ON.)

For the moment, that's all I've found. Please heed the warnings, and stay safe. Don't start deleting files all willy-nilly like I did, and cause more problems than there were before. Let me know if any of these fixes don't work, and I'll see what else I can do.

___

OTHERWISE.

HAVE FUN!!