Eistor is fairly easy to acquire, on my first playthrough I just prestiged every 15 levels after level 90 (105,120,135…) and was able to get him eventually. However if you’re having troubles check out this guide from chilip
I’ve recently restarted my progress on Tap Titans, and think I’ve found an easier way to reach Eistor during early prestige levels which require less “powering through” with better artifacts.
Step 1: Reach Hamlette. While doing this, level the other heroes as appropriate to receive pertinent passive skills.
Step 2: Level Hamlette to level 50.
Step 3: Evolve Takeda. You should be able to reach the gold necessary to evolve Takeda relatively easily at this point. It is, however, still a bit of a grind, especially the first 10 or so levels.
Step 4: Keep leveling Takeda. Once hitting level 25 or a bit past, you should have a real shot at being able to save up the gold to purchase Eistor.
Step 5: Purchase Eistor.