So I am DMing this Ap and we are in the 3rd book. The party just hit 10th level and is still tier 3 (rather than have characters gain tiers at different times, we are making the personal quest mythic challenges worth 1/4 a tier so the whole party will increase in mythic power at the same time.
Anyway, my party consists of:
Champion Ranger2/Paladin7/Bard 1, who uses two weapon fighting with a longsword and spiked shield to devastating effect (regularly does 120+ damage to demons per round)
Guardian/Heirophant Cleric of Erastil 7/ Hinterlander 3 who uses a Long bow with his wisdom modifier and casts buffs that last all day.
Trickster Rogue 8/ Arcanist 2 who is invisible at will and can never be detected.
Archmage Wizard 10 (Banisher/Abjurer) - who has flexible counterspell and spell sieve.
Now, a lot of the guides turn players away from Flexible counterspell, but the way my players use it, it makes enemy casters into punching bags. He already has such a large bonus to spellcraft checks that he almost never fails to identify the spell being cast and then shuts it down and casts it back. SO far, the only check to the power has been that some enemies can cast higher level spells than the archmage, but that is quickly going to come to an end, since he is a wizard and the campaign goes to level 20. I know the spellcraft rules state that perception modifiers also apply to attempts to counterspell, but there really isn't much written about what this looks like in practice. what effect does still spell or silent spell have on attempts to identify spells as they are being cast? Does invisibility add +20 to the difficulty of identifying a spell as it is being cast?
My party is having a lot of fun being the superheroes of the worldwound and I am glad that they get to dominate most of the time, but Most of the major villains in this campaign rely on spells to provide any threat to PCs and I want to be able to make smart enemies be capable of studying the party and not get completely blindsided by the "you fail to cast all spells" freight train that is flexible counter-spell.
If you are getting stressed at this stage about a fairly minor ability in this campaign you are in Trouble.
I ran this game to the conclusion and this is so minor on the troubles issues it is not worth mentioning. I will point out some solutions to this problem shortly but the Mythic rules are totally broken and once your pc's hit book 5 they will be able to kill Baphomet in a single full attack from your paladin so be prepared for trouble.
Counterspell requires a ready action so your wizard is locking himself down from doing anything else , this is actually a blessing for you if he keeps this up he will not be a big problem later (no Mythic Time Stop to isolate the enemy caster with your entire party, no massed wishes.)
Most of your major enemies are also mythic particularly the book 3 end boss so use this. At later levels they can take the 4 ranks of mythic ability which allow them to use metamagic for free and then cast a Quickened spell and a normal spell every round, this means at least one spell will go off and your wizard is not doing much. If you are using the RAW Mythic Haste cast that and then your wizards can cast 2 standard spells and a quickened spell (the pain the pc's will inflict upon you for allowing this broken spell is another issue I banned it), also RAW Arcane surge allows casting another spell as a swift action with a mythic point so any boss should be able to cast 2-3 spells a round which will quickly render counterspelling pointless and force your pc to break the game in an actually broken way.
I gave Deskari 16000 ho (yes 16000) my pc's killed him in 2 rounds, that is what you have to worry about
but flexible counterspell is an immediate action, so the wizard does not have to ready an action, although they can only shut down one spell a round.which means the wizard is still casting spells every round, often the best spell the enemy just cast at them.
I agree that the mythic rules are very challenging for the DM with smart players.
Quickened is the only way that any caster will be able to get a spell off, but rewriting every mythic caster to spend 4 mythic abilities just to get one spell off a round seems like a waste. I am more curious about how smart enemies, who have seen the PCs fight might plan for this contingency.
I know there are intrigue feats about making spells more difficult to identify but none of the characters in wrath were built with that in mind and it seems like counterspelling is such a rare part of most players experience that not much clarity has gone into defining what does and does not affect the spell craft check, as far as more common approaches a wizard might take, like still spell, silent spell, etc.