Spamming Toto-rak is by far the fastest way to make seals
There are a few guides out there but here's the short version to get all chests
1. Get all 6 chests
2. Open all gates
3. Kill all mites
4. Under 25mins
5. Gold chest from boss
There is also a "secret" to opening the chests at the end, I'm not going to post it but do some digging and ask around
Hope this helps