Well frankly I would say walk - it's only about 10-15 minutes maximum.
Garden Road is downhill only. Cotton Tree Drive is the uphill road. The 56 and 56A come up Cotton Tree Drive and into Robinson Road at the top. They're very often full at that point though so I don't think that's a route to rely on.
#12M is a different route from #12.
#12 stops at HK Park (outside the Squash Centre) and then takes the left exit off Cotton Tree Drive on and stops outside Kennedy Heights on Kennedy Road (which is about 150m down the hill from the YWCA) before going along Caine and then turning back up Seymour. The stop you need is the one at the top of Seymour Road, just before the back entrance to Wellcome.
But the #12 has a 20 minute headway, so really, at least 8 times out of 10, it would be quicker to walk. If you're doing it when the Botanical Gardens is open then go a few metres downhill from the YWCA, take the bridge across CTD/Garden Road, then head up and left across the Gardens to the top exit near the junction of Robinson Road and Albany Road, then straight along Robinson.