Alright, let us see...
Pikachu and Squirtle don't get much synergy anyways, so a cleric like Chimecho will definitely benefit you. Chimecho gets great supporting moves and access to the coveted IQ skill Nature Gifter, allowing him to heal your whole team with an Oran Berry.
Pikachu should run an anti-Monster House set. Discharge, Shock Wave, Grass Knot, Agility. Discharge murders Monster Houses, Shock Wave gives him a ranged attack, Grass Knot can hit Ground types that take little damage from his other attacks, and Agility lets him support Squirtle a little better.
Squirtle makes a great tank. Give him Water Pulse, Ice Beam, Protect, and Bite. Water Pulse and Ice Beam are good ranged attacks, hitting everything bar other water types pretty hard. Bite is probably your best melee option, and Protect lets him take on bosses and outlaws with ease.
Next, invest in a Chimecho. Chingling can be found in Drenched Bluff and evolved with a Lunar Ribbon. Chimecho should run Safeguard, Heal Bell, Calm Mind, and Psywave. If you're really dedicated, you can try to get a Chimecho with Wish, but seeing as Wish is an egg move, that requires a lot of effort.
Now, Pikachu hits hard, Squirtle can take hits, Chingling can support... All that's left is a decent sweeper. Darkrai immediately comes to mind...
Darkrai should run Dark Void, Dark Pulse, Dream Eater, and Focus Blast. He can be found in a number of dungeons after you acquire a Secret Stoneslab or Enigma Machine. The easiest is probably Miracle Sea BF3. Note: Bring a Golden Mask/Friendship Scarf and a Pokemon with the IQ skill Fast Friend. I believe Pikachu gets it.
Or, if you have a qualm with using Legendaries, Houndoom can do the same thing, but his offensive qualities are much lessened, and he doesn't get access to Time Tripper.
