What is a galaxy seed and setup to get the Immortal Bard achievement?

Try this:

seed: 184133953
Spiral-8,medium 2 empires,normal age
average system balancing,Medium galaxy density
Normal,Few
Average,Average
Normal,Normal
Normal
Started with Karta Vauclas
On turn 493 I finally got Tephys 

Taken from this Steam Community Post.

If you want to speed things up however, the same post also suggests doing an edit to the Hero.xml files found in either "\Steam\steamapps\common\Endless Space\Public\Simulation" or "\Steam\steamapps\common\Endless Space\Public_xp1\Simulation", depending on if you have the original game or the expansion. This allows you to remove all heroes except those two you need. Remember to take a backup of "Hero.xml" first if you want to revert your changes.

Also, any mods you have activated will disable you getting this achievement.

Hope this helps.