Battles with enormous creatures should be epic, yes? Some of the most exhilarating fights in cinema, literature, and video games have been arduous, grueling battles between our heroes and these cowering Titans who obliterate entire buildings without even meaning to.
Typically, in TTRPGs, these combats are scarcely differentiated from combats with medium sized creatures that have massive amounts of hit points. Sure, there are size categories that have certain implications, and there are often hard hitting attacks or special abilities. But there's a lot of potential for greatness here that I believe isn't being capitalized on.
For the sake of our purposes here, I'm going to be speaking in terms of 5e D&D mechanics, simply because it's incredibly easy to pare down to OSR standards, but is more widely understood.
TIP ONE: PARTITIONING HP
My first tip is to take that big ol' HP pool and break it down into parts. The objective here is to acknowledge the fact that combat is going to have an added dimension of scale thanks to the size of the opponent.
In terms of 5e, the easiest way to do this is thus:
1. Take the HP derived from CON modifier x Hit Die number.
In this case, if you look at the HP, see where it says 10d12+40? That 40 is your magic number. It comes from 10 Hit Dice X +4 CON mod. This 40 is your Vital HP. Vital HP covers everything from the chest, to the groin, to the crotch, to the head. Whatever is incredibly dangerous to take a hit in, that's considered Vital HP. When the creature runs out of Vital HP, they're mortally wounded or dead.
2. Take your hit dice, roll or average them, then divide that number by the number of active appendages your creature will be using. In the case of the Hill Giant, we have 65 HP divided by 4 (two arms and legs). We end up with 17 HP per arm and leg (I round up). When an appendage runs out of HP, they've been disabled. There should be mechanical ramifications to this to create a "death spiral" effect.
3. Assign a purpose to the appendages. We've already decided that Vital HP keeps them alive, so what does Leg HP do? What about arms?
For our Hill Giant, the legs correspond to movement, the arms correspond to blocking and attacking. When the first leg is disabled, I would reduce movement by half and have them roll a DEX save to not fall prone. When the second leg is disabled, they would be forced on their knees, spending an action to crawl with their arms or to only be able to shuffle 5 feet per round. These conditions can be reversed by anything as measly as 10 minutes for a quick regenerating creature to a full night's rest.
As for the arms, you need to decide if there is a dominant hand. You can actually telegraph this to the players if they ask which arm the creature seems to favor. If there is a dominant hand, disabling means that the attack rolls using large weapons should be made at disadvantage, AND there should be an AC decrease overall. The reason for this is that if they plan on using their weaker hand to attack by itself, it will be less effective and they will not be able to use that hand to defend themselves as well.
If both are disabled, the Vitals should be more exposed (more AC loss) and they'll have to use kicks or bites to attack. Getting a PC who is climbing them off of them might mean rolling around on the ground or slamming against objects.
The next suggestion goes hand in hand with HP partitioning, giving players that sweet, sweet opportunity for risk vs reward.
TIP TWO: AC PARTITIONING
So, we've broken our monster into pieces for HP and role purposes. Now it's time to find out how hard these parts should be to hit.
When you're dealing with something that anatomically resembles a human, defense of the chest and head is by far the easiest to defend. Why? Because of the arms. Our arms serve the purpose of defending our vulnerable areas, notably our eyes, throat, groin, etc. Because of this, I would do the following.
1. Vital zones have +6 AC (adjust as needed per system) added to the default as long as both arms are still in action. For the Hill Giant, this would be an AC of 17. From a probability standpoint, your average level 1 PC would have a 40% chance of hitting.
2. Arms have a +2 AC each. Disabling an arm drops Vital AC by 2. Again, for average level1, 50% chance.
3. Both Vitals and Arms require that the PC's attack is able to reach that zone, whether by ranged, flight, or climbing.
4. Attacks that bypass AC such as Magic Missile or features that use saving throws instead will target one of the arms instead. If no arm can come to the defense, the Vital area is targeted.
5. Legs have no inherent AC bonus, so for an average level1 PC, they'd have a 60% chance of hitting and could attack them from melee on the ground. Once the legs are disabled, the Giant is unable to stand, leaving their arms and Vitals vulnerable to ground attacks.
6. Climbing a body part and attacking it means that there is no bonus to AC when you make that attack, as they cannot defend themselves properly.
TIP THREE: ACTIONS
If you're using a Giant creature as a solo monster against the party, it would be wise to consider extra actions, even if they don't have them in the stat block. However, these extra actions would be primarily defensive in nature and limited to 1 per turn.
Perhaps every round, the Giant should be able to attempt to swat off someone who is climbing them. If they're just taking a swipe at them, perhaps it should be an attack roll at disadvantage to knock the PC off of them. Perhaps the Giant could belch or roar in their faces, exposing a close PC to a CON/WIS save to see if they fall victim to retching from the hideous odor of the creature's jowls or are filled with dread for a turn.
A full action that should be possible as well is rolling over when the Giant is on fire or being climbed on by opponents. I'd consider being rolled over by a giant to be d6 worth of damage (scale for CR as needed) if the PC doesn't jump off and a Skill Check to see if they stay on, but I wouldn't go any higher than that, as I wouldn't want this to become a primary means of attack.
Finally, you should tweak the existing attacks to have more gravitas. For instance, if a Giant does more than 1/4 HP of damage with one attack, the PC should be knocked back 5 feet for every 10 HP of damage. If they narrowly miss their target who is hiding next to a building, the building should get smashed in. Anything that you can do to demonstrate the potential for wanting destruction from these hulking badasses is a net positive.
If they're dealing with a sufficiently large Giant, I would definitely consider a near miss to require a Saving Throw (DC 10, perhaps) to not fall prone. The impact from an enormous club smashing the ground right next to you could cause a mild tremor, or throw a strong wind in its wake.
TIP FOUR: BRINGING IT TO LIFE
Don't forget that as the DM, you're responsible for painting a picture for the players. While you don't want to drown them with details and bore them to death, you also don't want to be so scarce with details that they feel as though they're just fighting a stat block.
To really make this thing feel real to them, the first thing you want to do is give them a sense of scale. Don't just say "It's a Giant," let them know approximately how tall this bad boy is!
Anything from "the eyes of (tallest PC's name) are barely even with the midpoint of the Giant's lower leg" to "The hulking figure before you stretches well over 25 feet high!"
This helps them picture things better, but it also serves a mechanical purpose. How many rounds does it take for the Fighter to climb up to the head of this creature? What about the Thief, who suffers no movement penalty with their climbing subclass feature? Now you know.
Similarly, if the Wizard casts a spell that envelops the Giant entirely, guess how which parts are subject to the effects? Yup, all of them. For the sake of damage, I'd have them roll it and divide it equally among all the parts.
A Giant, in my estimation, would also be exceptionally loud when trying to speak to the PCs. Try to convey the loud bellows, the rancid breath, the awful hygiene when they lift their arms. A PC crawling too close to the armpit should definitely suffer for having done so. If the Giant is docile, perhaps they try to pick up a PC to try to talk to them, like we would pick up a mouse or an infant.
WRAPPING IT UP
If you need this advice, even in spirit if not in letter, you're going to have more interesting fights with Giants. Players will be able to plot their moves with more tactical thinking, in a very intuitive way.
The purpose of all of this isn't to add complexity for complexity's sake, but to give players a way to make more meaningful choices in these scenarios. They'll know that when they fight your Giants, Fly is an invaluable spell, climbing to stab in the chest can be a worthwhile risk, marksmanship seriously counts, and if nothing else, you can take them down bit by bit and force them to surrender.
I hope you've enjoyed this entry! I'll be doing another Pedantic Pen and Paper Podcast with my wife on Tuesday night at 6:30 PM. Feel free to drop by and leave some questions (which you can send to email@example.com) so that we can answer them in the stream.