CR vs PC lvl?

[Jellyburd]

need some quick advice,
Im setting up for a session tonight and was planning on using a CR 7 Grick Alpha
against 4 level 5 PCs. This could possibly be after up to 3 small encounters with regular Gricks.
Kobold fight club rates it a medium challenge, however it can do some gnarly damage 4d8+4 with tail. insta kill on the squishy PCs.
It also has alotta resistances but only 75 HP.
Should I trust Kobold or the CR?

[MaxWilson]

need some quick advice,
Im setting up for a session tonight and was planning on using a CR 7 Grick Alpha
against 4 level 5 PCs. This could possibly be after up to 3 small encounters with regular Gricks.
Kobold fight club rates it a medium challenge, however it can do some gnarly damage 4d8+4 with tail. insta kill on the squishy PCs.
It also has alotta resistances but only 75 HP.
Should I trust Kobold or the CR?

Kobold rates it as a not-quite-Hard challenge, so Kobold and the CR are not as far apart as you might think. The 4d8+4 attack (tentacles, not tail, BTW) might reduce someone to 0 HP, but it's probably not an insta-kill unless they have extremely low HP for their level. If your party is of average or better toughness for their level, it should be fine, as long as the PCs are not surprised by the Grick Alpha.

[Fire Tarrasque]

If you have Xanathar's, it should help you. It gives appropriate CR for level encounters.

[Asmotherion]

Depends more one other things:

-The Players level of experiance.
-Avalability of Healing between the Encounters; Even a Short rest before the Boss Fight can make a Diferance.
-Co-Ordination between Players. If players argue in combat instead of going for strategic play, then don't opt for "Deadly Boss Fights" rather than "Thematic/Rp intence Boss Fights". If the Players work well together, level 5 makes a huge diference in power level from level 4.

For referance, with a party of 4 well experianced players playing level 5 characters, we made it through a dungeon with:

-20+ Kobolts
-20+ Zombies
-1 Human Wizard
-1 Half Elf Bard
-1 Beholder

Not everything should be expected to be solved with brute violance though; Some aspects of the dungeon should be optional, skipable, "trap options" or just lead you to a dead end. Remember you're not playing a Computer RPG/MMORPG were the most complex quest you can give sums up to "kill 12 wolves and bring me their pelts" or "kill the boss of that dungeon". You can trap a few Kobolts in their chambers, poison their drinks by charming the guard and persuading him that "this will make the Ale much better, worthy of the Drinks of a True Dragon" or just try to sneak past all encounters and (if DMed fairly) risk a chance of facing everything together in the end by a percentage die.

[Nidgit]

Seems like a decent challenge. If you're really concerned about OHKOing a PC, just have the Grick Alpha attack a beefier one first so the players get an idea of the damage it can dish out. Even with an especially frail character (Sorcerer/Wizard with +1 CON or less), they won't be downed in one shot unless they're especially unlucky on the damage roll.

[Requilac]

Are you just using the grick alpha alone to fight them? If so, then I wouldn't really be all that worried about CR. If it is a lone monster then you shouldn't have to worry about it killing too many players unless it is of a drastically higher level than they could take. Action economy and all that makes it hard for "boss" monsters to really be a challenge. I wouldn't worry about it all that much.

[Ronnocius]

need some quick advice,
Im setting up for a session tonight and was planning on using a CR 7 Grick Alpha
against 4 level 5 PCs. This could possibly be after up to 3 small encounters with regular Gricks.
Kobold fight club rates it a medium challenge, however it can do some gnarly damage 4d8+4 with tail. insta kill on the squishy PCs.
It also has alotta resistances but only 75 HP.
Should I trust Kobold or the CR?

Like Requilac said, solo monsters are usually not very challenging even if the party is lower level. I would give them some clues that a bigger monster is up ahead, so they can make sure they are rested/prepared to fight it, just in case.

[JNAProductions]

According to the DMG, it's just below Hard. So it should be a reasonably challenging encounter, but unless the Grick actively tries to kill those who it downs, shouldn't be a big deal.

[Lombra]

At least one will go down, if dice roll poorly it could be a tpk. You are right about being worried of that attack. It's always swingy when you use monsters of a CR higher than the party's average level. Give them a chance to rest before the encounter.