I know of a few ways to do this, but all require substantial investment of character resources (class levels and spell access); they're not something that just any character can pull off.
The Enlightened Fist PrC (CA, p. 34) gains the Arcane Fist ability at 3rd level, which reads:
Beginning at 3rd level, an enlightened fist can spend one of her daily stunning attempts to cast and deliver a touch spell as part of an unarmed full attack action. She can choose to deliver the touch spell with any single unarmed strike attack she makes during the action.
Since this ability doesn't restrict spells based on their casting time, and casts the spell as part of the normal full attack action, it can be used to quickly cast spells with long casting times (though not quite as a standard action). Liquid Pain, as a touch spell, works with this ability without further modification.
Imbue Arrow + Greater Glyph of Warding
The Arcane Archer gains the Imbue Arrow ability at 2nd level, which reads (emphasis mine):
At 2nd level, an arcane archer gains the ability to place an area spell upon an arrow. When the arrow is fired, the spell’s area is centered on where the arrow lands, even if the spell could normally be centered only on the caster. This ability allows the archer to use the bow’s range rather than the spell’s range. It takes a standard action to cast the spell and fire the arrow. The arrow must be fired in the round the spell is cast, or the spell is wasted.
Since this ability always takes a standard action to use, but doesn't care about the casting time of the spell, it can be used to quickly cast spells with long casting times.
Unfortunately, Imbue Arrow only works with area spells, which Liquid Pain is not. To solve this, we use Greater Glyph of Warding in its Spell Glyph configuration, to create a glyph that casts Liquid Pain on creatures within its area.
Greater Glyph of Warding is normally a Cleric spell, but if you want to avoid splitting up your caster levels, you can get it on an arcane caster by taking levels in Geometer (CA, p. 39)
This method isn't 100% RAW airtight - it depends on how broadly your DM interprets the sentence "You set the conditions of the ward" from the spell description of Glyph of Warding. You can read some discussion of whether this works in this question.
Spellsword's Channel Spell
The Spellsword PrC (CW, p. 79) gets the Channel Spell ability at 4th level, which allows you to cast a spell into your weapon with a move action, the discharge it against an enemy the next time you hit someone with an attack:
At 4th level, a spellsword can channel any spell he can cast into his melee weapon. Using this ability requires a move action, and the spellsword uses up the prepared spell or spell slot just as if he had cast the spell. The channeled spell affects the next target that the spell sword successfully attacks with his weapon (saving throws and spell resistance still apply). Even if the spell normally affects an area or is a ray, it affects only the target. The spell is discharged from the weapon, which can then hold an other spell. A spellsword can channel his spells into only one weapon at a time. Spells channeled into a weapon are lost if not used in 8 hours.
Like the above methods, this ability doesn't care about the casting time of the spell to be channeled, allowing for accelerated casting.
It does have a couple of drawbacks: First, it's sharply limited by only being usable a few times per day. Second, since you have to spend a move action to channel the spell and then make an attack, the overall action cost will typically be the equivalent of a full round action, rather than a standard action.