Assuming that you have enhanced all of your character's gear to +15 by doing the process below.

Above are the failstacks you can use when enhancing your gear depending on the level. Going from +15 to PRI is easy. You'll experience how cruel the black spirit is at DUO and TRI. Try using your dummy gears to make failstack but this time you need to fail the dummy PRI gear as well because PRI gives you +2 fs per fail, DUO +3 per fail, TRI +4 fs per fail, TET +5 fs per fail and lastly PEN gives +6 per fail.

Here is an example below where I failed dummy PRI gear 2 times in order to get additional 4 FS to add to my existing 20 and then I failed a DUO to get 3 more fs. Now with 27 FS Obviously you don't want to use it at PRI gear because the FS is to high for PRI, it's your decision if you want to use it to enhance your DUO to TRI or fail another DUO to get additional 3 FS to make it 30, remember it's RNG 27 can still succeed to TRI. This is the idea the I want new players to have right away. Higher failstacks requires higher gears to fail.

That's why I recommend you to have a lot of dummy green gears because sometimes you will succeed and if that happens then you can no longer use that dummy gear for lower fs and you'll be needing another one.

There are items that can make your failstacking easier.

1. Valks Cry. This item gives you +1 enhancement failstack but you can only use up to 10 valks cry for +10 fs. (NOTE: I recommend Using it only when you have 40+ fs or above.)

2. Advice of Valks. This will grant your character instant failstack. If you have +50 Advice of Valks you get 50 fs upon using this item.

3. Cron Stone. Once you enhance your gear up to DUO it is now possible for that gear to downgrade upon failing. Cron stones prevent gears from downgrading.

To get easy 30 FS some of the players make 20 fs using reblath/militia then they use Valks Cry to add 10 fs. Some of them use valks cry at 40 or 50 fs so they can reach a TET fs. It's really up to you.

Get more Valks Cry and Cron Stones by purchasing oufit from Pearl Shop and make your life easier. Each of the outfit parts contains Valks cry and Cron stones you decide what you want to get just go to a blacksmith and extract it.

Remember to use failstack to the right enhancement level you don't wanna use 70 FS to enhance your PRI to DUO because that 70 fs can be used for TRI going to TET enhancement level. Make sure to fail fail and fail but don't get too greedy so you dont succeed and waste your fs. Once you have the desired FS go ahead and switch the gear you really want to enhance and hope for the best.