Hey Trit! Good to see you on here again!
I'll tackle this one. First, I'll tell you right off that bat that it's not CR 5, probably more like CR 9 or 10.
Spell Reflection is a tricky trait, so our first step is to reverse engineer the
spectator (beholder) to get a read on how much Spell Reflection influences Defensive CR and/or Offensive CR.
Step 1: Reverse engineering the spectator's CR
At 39 HP, and AC 14, the spectator's baseline Defensive CR = 1/4.
To determine its 3-round DPR, we'll assume it uses Wounding Ray twice per round. While this isn't strictly accurate, it's a fair approximation since paralyzation and confusion are pretty significant control (single-target fear less so). So that is 16 + 16 = 32 damage. With save DC 13, that means its baseline Offensive CR = 4.
Now, the spectator is described in the MM as a CR 3 monster. So the designers are interpreting Spell Reflection to boost either its Defensive CR up to 2, or its offensive CR up to 6, or – more likely – boosting Defensive CR to 1 and Offensive CR to 5.
So that's clear how Spell Reflection is interpreted for the CR 3 Spectator. But let's go one further step and evaluate
what kind of magic the spectator is likely to be using Spell Reflection against. Well, it is CR 3, so the default assumption is a party of 3rd level PCs would be facing it. A typical 3rd level damaging spell might be
shatter which deals 13 (3d8) damage. However, a 3rd level wizard only gets two 2nd level slots, so...
What if we increase its effective HP by 2 * 13 = 26? And increase its DPR by (13+13+0)/3 = 8?
Defensively, it now has 39 + 26 = 78 effective HP, and still AC 14, which makes its Defensive CR 1/2.
Offensively, its DPR now is 32 + 8 = 40, still save DC 13, which is Offensive CR 5.
Making it's total CR 2.5, or rounded up to 3.
Step 2: Applying this to the Obsidian Golem
It's approximately CR 5 – or that's what we're aiming for – so that means typical damaging spells it will be using Spell Reflection against are
fireball or
lightning bolt, which deal 28 (8d6) damage. Again, however, the wizard only has two 3rd level spell slots. So, I'm going to increase its effective HP by 2 * 28 = 56, and I'm also going to increase its DPR by (28+28+0)/3 = 18.
Step 3: Ironing out the kinks with "Sharpened Form" trait
Sharpened Form. Whenever a creature within 5 feet deals 20 damage or more to the golem in a single round of combat, a shower of glass, a shower of glass erupts, dealing 8 (2d6) slashing damage to the attacker.
Your intent with this is unclear.
Do you intend that, say, a spellcaster could hit the golem with a
shatter spell and...nothing would happen?
In other words, do you only intend for this to trigger when a
5-ft melee attack hits the golem?
Also, you say "in a single round", but that would include off-turn opportunity attacks. Do you mean "in a single turn" (a cleaner way of also including opportunity attacks without the ambiguity)? Or do you actually mean "on that creature's turn" (hence excluding opportunity attacks)?
These questions will affect how often Sharpened Form gets triggered.
I'm just going to assume that it gets triggered once per round, thus increasing the golem's DPR by 8.
Step 4: Golem Maths
Now, the golem's
Defensive CR = 11
Effective HP = (85 * 2 damage immunities for a CR 5-10 monster) + 56 damage foregone thanks to Spell Reflection = 226
Effective AC = 16 + 2 magic resistance = 18
And it's
Offensive CR = 8
DPR = (13 + 13) multiattack + 18 Spell Reflection + 8 Sharpened Form = 52
Effective Attack = Should be +7*
*your stats list +6, but with Strength 18 and being a supposedly CR 5 (prof +3), that should be +7.
Thus, it's
total CR = (11 + 8) / 2 = 9.5 which you can round up or down as you see fit.
Because it's a CR 9 or 10, you can still assume the x2 modifier for damage immunities. However, it's proficiency modifier is +4, so that will put its attack bonus at +8. That's not enough to change its offensive CR, but just want to make sure we get it right.
Final Thoughts
Because this is a golem that's more likely to face PCs of 9th level, you may wish to reevaluate how significantly Spell Reflection influences its CR. I made the assumption that the PC's offensive caster only had two 3rd level spell slots to throw
fireball / lightning bolt at it twice during combat. However, if the party has 5th level spells, that could change things. Personally, I think just rounding it up to CR 10 would be good enough.