Here's how you go about answering the question of which eidolon is better (for you!):
- Look up the specific percentages for each skill in the buffs/debuffs section of the JP wiki
- Go to my Toolbox and punch in the numbers for Team 1 with Fafnir in the Assault and Eidolon comparisons tab, then Team 2 with MLB Typhon. If you don't have MLB Typhon, add the difference of their Attack stat from Team 1's attack power to determine the team attack power of Team 2.
- Use the Adv DMG calculator to factor in the eidolon ATK buffs. If you're merely interested in the added spike damage, set "Turns without Burst" in the Assault and Eidolons comparisons tab to 0. The simulation assumes that ATK buffs are used to buff DMG skills and Bursts.
- Get a result and know which one is better based on your situation. You can further customize the Team on the Team customization tab, which is currently set to my team of characters by default.