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See when a monster, this is all my opinion btw, is just grabbed whole out from any monster manual it is DM trying something new or DM panic mode where the toughest thing is supposed to take care of a party that is roflstomping encounters.
Huh? So, you never use stock monsters? By which I mean a monster just as written in the book. That's weird, b/c I use them like 90% of the time. I may throw on a few feats to change up their fighting style, or a template, but that's only about half the time in the games I'm usually involved in.
Indeed, one of the great advantages D&D has for a GM, as opposed to say Godlike, White Wolf, Mutants and Masterminds, Star Wars Saga Edition, Burning Wheel ... ad nauseum, is that it has a wealth of pregenerated enemies and some (highly imperfect) guidance as to who they represent a challenge to. You're suggesting that it's odd to use that one big advantage it has.
The situation here is a Solar tasked to guard an area. "Stock" Solar would not be used because this is a special encounter due to the nature of what is said. If I was DM and made this up, against a party of four, Solar would have level appropriate gear, different spells, and maybe some skill changes. This is either a boss challenge or a mini boss challenge not group of El 23 characters rushing a celestial place where Solars are common guards. If the Solar is supposed to be a common guard, which is how I see something EL 23, then yes the Solar from the MM is fine because it is not supposed to be specific. But this is one Solar guarding a special place against EL20 characters. So yes it would have gear and other things tailors to protecting an area and not "stock" monster things.
Henceforth nerfing "stock" monster Solar, to me, doesn't make this a worthwhile challenge due to how you designed it. Because it would summon things to aid in patrolling because it wasn't given orders otherwise and it is rather smart to do so. Heck if this was a written scenario the rogue would have to go through a whole lot more, waste more resources, before meeting the Solar.
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This thread was, I believe, spun off from a "rogue's suck" type of argument. I don't see how any of the above quote even jives with D&D, or any useful exercise, though. First off, the stock Solar, meaning the one in the Monster Manual without any changes, is supposed to be an ok encounter for a group of level 20ish characters. That's what CR means. You can debate its accuracy, but that is what it's supposed to mean.
It's obvious that a powerful monster given sufficient gear, help, and set-up could prove really really challenging against whatever party or characters can be conceived. But, all that's been said there is that "it is theoretically possible for the DM to challenge such and such group of players and characters." Unless we're in hard-core TO territory, that strikes me as sufficiently obvious to be assumed. Call it an axiom.
If you were to give the Solar 100,000 gp worth of gear, allies (beyond its summoning capacity, see below*), customized spells to counter the particular party's abilities, you'd be raising its effective CR. Now, that might be fine, it's CR might be low-balled or whatever anyway, but that is what would be going on. There's no way that adding a template like half-dragon, which increases its stats and so on, increases its CR while giving it magic items that could have literally the same effect wouldn't. This was made more clear in later monster manuals as specific combat gear was included and separated from treasure.
In short, you can't just say "it's a high level encounter, so I'm going to give this already high level monster additional stuff to make it challenging" and call it a day. Or, let me be clear, you totally can (and probably should, to be honest) do so in your campaign. But, if we're testing whether a build or tactic is viable, then you shouldn't.
*That being said, I would fully expect a Solar set to guard duty to summon all sorts of patrolling minions to the limits of its ability or ease. That must makes sense. Especially since the Solar has better things to do than worry about every stray goblin that happens by, like contemplating the nature of creation or something. Frankly, I find that much more plausible, given the implied personality of these guys, than them hanging out invisibly for ages on end.
If I were asked to judge the power of a Solar vs. a build I'd use the stock one, including all of its abilities and spells as written (i.e., not changed to foil any particular tactics), or perhaps changed slightly to give it some guarding type spells -- the Solar knows that's what it's job is going to be. The only one I might set aside is Miracle b/c it's too open-ended. Personally, I don't think it's super helpful since it's a caster monster, so it's almost like just judging a high-level cleric v. a high-level rogue, so I would have gone for a different opponent. But, that's just my humble opinion.