So I made a monster for D&D 5e. It's a multi-elemental elephant called the Elementalphant and its CR comes out to be about 8-9, but that kinda seems low. If I were to throw it against my party of six level 5's it would destroy them in only a few turns, without taking much damage. Here are all of its stats:
HP 178 (20d12+48)
Resistance to fire, acid, lightning, cold
Immune to poison
Move speed 40ft, fly 20ft
Gore +7 to hit 3d8+7 piercing + 1d6 lightning
stomp +7 to hit 3d10+7 bludgeon + 1d6 force, only on prone targets
fire breath 30ft(from mouth) dc 16 dex save 10d8 fire 5~6 recharge
cold breath 30ft (from trunk) dc 16 dex save 10d8 cold 5~6 recharge
lightning "breath" 120ft line (from tusks) dc 16 dex save 10d8 lightning 5~6 recharge
Multi attack Gore x2
Acidic blood: when slashing damage is dealt to this creature a small spray (5ft) of blood deals 1d6 acid.
Trampling charge: if this creature moves 20ft in a straight line then uses Gore target creature needs to make a dc 16 str save or be knocked prone, stomp can then be used as a bonus action.
Quaking stride: if a creature is within 10ft when this creature moves they must make dc 13 dex save or be knocked prone.
So what should be its CR? I want to throw this monster at my players, but I don't want to wipe them out and I want to give them a fair amount of XP when they do eventually kill it.