My main concern is getting the right damage for claw and bite attacks without making him too easy and just a bag of HP. I don't like the DMG's approach of trial and error. I want more steamlined monster building rules instead of the "fly by the seat of your trousers" ones. I'm not looking for him to be a solo per se, he is always followed by a pack of orc ghouls.
Describe the player party to me? #s? Average level? How optimized? How skilled are the players? Do they have a cleric with Turn Undead?
Is the hill giant ghast to be encountered solo or with other monsters? Should it be legendary?
Off the top of my head: Combining a Hill Giant (CR 5) and a Ghast (CR 2) – applying a "ghast" template if you will – seems orders of magnitude weaker than a CR 16 creature. CR 16 would be a mummy lord, an adult blue dragon, an iron golem, or a marilith. Nothing wrong with changing the narrative, but if your PCs later fight an adult blue dragon and the players think "this was about as hard as fighting that ghast hill giant", then that says something about your world. So, there's a world-building element to this as well. For example, instead of describing it as a hill giant ghast, you might devise a world-specific undead giant (e.g. in Al-Qadim this would be "rom") which haunts very specific crypts in the desolate wilderness...they're believed to be myths by most living, except for a few elves who recall their forefathers fighting the giants and cursing them.
Specific example... you'll notice this is a mostly straight translation of the concept with greatly exaggerated HP, and even then attaining CR 14 is about the highest I could stretch the concept. To take it that next step to CR 16 you'll either need to inflate numbers, and/or start adding new abilities/traits/actions to the concept, and/or make it legendary.
Hill Giant Ghast
Huge undead giant, chaotic evil
AC 15 (natural armor)
HP 248 (24d12+92)
Speed 40 ft
STR 22 (+6) DEX 14 (+2) CON 19 (+4) INT 7 (-2) WIS 9 (-1) CHA 6 (-2)
Damage Resistances necrotic
Damage Immunities poison
Condition Immunities charmed, exhaustion, poisoned
Skills Perception +4
Senses darkvision 60 ft., passive Perception 14
Language Giant
Challenge 14 (11,500 XP)
Horrific Stench. Any creature that starts its turn within 15 feet of the hill giant ghast must succeed on a DC 15 Constitution saving throw or be poisoned until the start of its next turn. On a successful saving throw, the creature is immune to Horrific Stench for 24 hours. If a creature fails its saving throw by 5 or more, it falls prone and is incapacitated while it is poisoned in this way.
Turning Defiance. The hill giant ghast, and any ghouls or ghasts within 30 feet of it, have advantage on saving throws against effects that turn undead.
ACTIONS
Multiatttack. The hill giant ghast makes one bite attack and two claw attacks. It may replace any of its claw attacks with rock throws or grapple attempts.
Bite. Melee Weapon Attack: +11 to hit, reach 5 ft., one creature.
Hit: 34 (6d8+6) piercing damage. Against an incapacitated target or a target grappled by the hill giant ghast, a bite attack is an automatic critical hit. If this bite reduces a creature to 0 hit points or less, the hill giant ghast tears off one of the creature's limbs and devours it.
Claws. Melee Weapon Attack: +11 to hit, reach 10 ft., one target.
Hit: 27 (6d6+6) slashing damage. If the target is a creature other than an undead, it must succeed on a DC 15 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Rock. Ranged Weapon Attack: +11 to hit, range 60/240 ft., one target.
Hit: 22 (3d10+6) bludgeoning damage.
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Quick monster maths...
Defensive CR = 13
248 hp >> CR 12 base
Effective AC 19 = AC 15 + 4 due to Horrifying Stench >> bump CR up 1
Offensive CR = 15
Damage-per-round 88 = 34 + 27 +27 >> CR 14
Effective attack +11 >> bump CR up 1
Total CR = 14 = (13 + 15) / 2
