The biggest issue you face; is that Brussian does not learn Werewolf Bite till Lv56. The Werewolf status effect boosts Cindy’s Speed stat by a pretty substantial +56%, and that is not something that can easily be overcome or replaced. I believe that reaching Lv56 with Brussian to learn Werewolf Bite; would likely be faster than leveling up Cindy enough to succeed without it.
For what it is worth, the original Lv50 requirement that said you needed the Lindertz Scimitar, Sharp Spike Shield, etc., did not take the Lust Power (+10% Speed) status effect into consideration. So you do have some wiggle room to work with, and no longer need the absolute best Speed+ gear anymore. Still, the 56% speed boost from Werewolf is probably irreplaceable, short of leveling Cindy up to maybe Lv80+ perhaps? (Maybe Lv65+ if you are on Easy Mode? I’m really just kinda guessing here though, no guarantees)
You may also consider just waiting until LF3 to open this chest. There are additional speed boosting status effects available by then; that make it much easier to reach 65 Stealing. Granted, you would not be able to get the Duke’s Monocle, or complete the Animal Butchery Quest until LF3. I am unsure if there are any other side effects to waiting until LF3 beyond that.
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Also, here is the stealing information that was added to the 2023 edition of the walkthrough. It says pretty much everything that can be said on the topic in excruciating detail, and will potentially answer any follow up questions you might have:
The exact formula to calculate your Stealing seems to be: (Credit to Seraphus)
+18Pts: Stealing Points: A reward from the Thieves' Guild in Chapter 3 of LF1.
+8 to +12Pts: Primo Hat: Equip on Cindy. Import exclusive. (Bonus is random each try)
+1 to +3Pts: Luck Ring: 'Other' equipment for Random. (Bonus is random each try)
+1Pt: Per 10 level ups on Cindy (So +5Pts at Lv. 50 for example)
+1Pt: Per 100 speed on Cindy (So +10Pt at 1000SP, or +15Pts at 1500SP for example)
(The Primo Hat and Luck Ring offer a random bonus each time. The two together will give you +9 to +15 Stealing on any given attempt, so multiple attempts will be needed to get +15.)
Status Effects that boost your SP stat:
Easy Mode: +30% SP (Select Easy Mode at the start of the game)
Lust Power: +10%SP (Give 5000 gold to Magdalena. Access with high GWP at Harpy Guild)
Werewolf: +56%SP (De-equip Brussian’s weapon, so Cindy survives Werewolf Bite [Lv. 56])
Energy Burn: +100% SP (Tarabulla’s Energy Burn; inflict Werewolf first as this halves Max HP)
(To get rid of Werewolf after, de-equip Cindy’s weapon and stall out a fight by using the Defend command for several rounds in a row)
To reach 65 Stealing you have to boost Cindy’s SP stat considerably. Equip the best SP+ items you do own on Cindy, and stack every Aura listed above that you can access. Assuming Cindy is Lv. 50, and you have +18 Stealing Points from LF1, and get the full +15 Stealing bonus from the Primo Hat and Luck Ring; Cindy will need 2700SP to succeed. If you don’t have the Primo Hat, but do have everything else, Cindy will need 3900SP to succeed.
Cindy doesn’t actually need to be level 50 to achieve this anymore, since I found out about the Lust Power aura, but it does give you some extra wiggle room to work with. You can also use some of your permanent +SP items on Cindy to reach the next 100SP barrier, if you’re right on the cusp of success.