Need ideas for dungeon area

[ArtenDaelus]

So i am currently DM-ing a PF campaign and we have a session tommorow.
The main problem i am not sure what to put up against my players since they can survive everything.
The players hit level 4 last session,and are now entering a crystal caven that is full of corruption.
The main boss for the area is a CR 7 Crystal Beast Abomination (Dire Boar/Elk Megaloceros) i am thinking that it will be too weak but i am not sure,one thing i forgot to mention everything that is non human is one size category larger because of the enviroment.
Should i add something else to the beast or leave as it is?

Thats for the first part
The second part is the campaign has artifacts scattered all around the world that effect pc and npc
And one of the players found a inert artifact that adapts to the players needs.
The artifact is 7 feathers with each having 5 different abilities but i am not sure what to put for the abilities and what should the requiremets be?

[Geddy2112]

A single boss is never that good of an idea. It only has one turn vs the party which has turns=# of players. So to make a boss able to live past a turn, it is either really hard to hit or has boatloads of HP. The former makes the fight near impossible, the latter makes it a grind(past 8 rounds for anything but a BBEG level boss is normally too long in my experience, YMMV). The other thing is that it can be so dangerous it can instagiblet PC's in a single round.

The CR system is a bit flawed, particularly with an abomination which can become far more powerful than CR+1 or far less. A megaloceros and dire boar mixed together is probably not justified as a full CR increase IMO. While a crystal creature could be very tough or almost unbeatable depending on your party makeup. Resistance to elements, flat DR, light based spells, and being a mindless creature negate a lot of things a lot of classes can throw at it. The blindness is also going to be tough as characters don't have access to the spell remove blindness, even if the DC is low. If you add anything, add a CR-1 template or a CR+0 that maybe gives it some weaknesses, as it is going to be tough as is. Now if the party has a bunch of bad touch spells they might nuke the thing, another reason CR is not very accurate. It would be best to know the # of the party, their builds and relative strengths/weaknesses.

Consider adding mooks to make the fight more interesting and preventing the party from abusing action economy. The mooks should not be something that can be ignored-they need enough HP to survive a hit, the ability to hit at least sometimes(not just on a 20, maybe a 16-20) and do enough damage they can't be ignored, but their strength should be numbers never that they themselves are powerful. Advantageous terrain for the boss is also helpful.

[ArtenDaelus]

Thank you for your advice
The party consists of :
Wolf Druid
Gunslinger
Spell Sage Wizard
Stonelord Paladin
Undead lord Cleric
And a Mesmerist