Keep us updated on this!
> wants lasting bruises
Well, there's wanting and there's what's possible! Some people bruise easily - with a touch - on some parts of their bodies, while others can't without some heavy severity, or possibly at all. So unless Jaro has previously had lasting bruises, there's a chance that isn't possible.
For lasting bruises, my initial intuition is to use warmth as prep and cold as aftercare: warmth will get the blood flowing, getting it into the area and could make it more sensitive to pain. Then do anything You like to Jaro. Afterward: cold will both reduce felt pain (delay it), but could also restrict blood flow, thereby having a chance to 'trap' more blood in the area for longer. Maybe teach it how to linger..?
As for implements, there are many. There was one kink night at a bar where this Dom was spanking people with a STOP sign. A 4-foot metal octagon on a 4x4 wooden beam. It was wild even to think of, let alone witness! More generally paddles with holes can produce the most wide-spread effects, and for some, bruising, if the person likes thumpy-with-some-sharp-accents.
Unfortunately I'm not skilled in marking an ass... well, not strictly true. I don't have any practice in reliably marking general/unknown asses. I just keep going until I'm done
But maybe we can learn from other things: not strictly applicable, but during a 'biting' phase I would linger, tasting and licking beforehand, and once I took hold with my teeth I would effectively 'massage' her with my teeth - both going deeper, giving the skin and muscle and fascia time to relax into it for several minutes, and thereby mark with deeper colors and physical impressions for much longer than a brief nip would...?
Happy experimentation! Keep notes!