Step 1: Convince your DM to let you play one. Congratulations! You now have an optimized character.
Seriously, though. The whole Synthesist thing is about jacking up your stats and giving you abilities you could never have gained before. The really cheesy tactics involved taking a venerable character and tanking all of their physical stats to jack up the mental stats, then just use the physical stats of the eidolon. Build like a melee bruiser, likely with a lot of natural attacks. High Strength, Power Attack, beat things in the face.