Ways to Get a "Guaranteed" 4* Champ

1. Save up units by farming quests, arenas, event milestones. After pulling a 3*, the "special offer" for a 4* crystal pops up (cost of around 2500 units, offer lasts 24 hours.)
2. Fully explore, not just complete, the entire story quest line for a guaranteed 4* crystal.
3. Rank high enough in the 4* arenas (this is near impossible for me, I played every 2 hours for 3 days straight once and did not get anywhere near the cutoff with 18 3* heroes.)

Others? I still only have 1 Looking for other options...