Solaniwa Onsen is connected by a direct walkway to Bentencho Station, which is around 10 minutes from Osaka Station (Umeda Station) on either JR or Osaka Metro.
You can also use the parking lot in Osaka Bay Tower, the complex that houses Solaniwa Onsen. (There is an additional fee.) Please see the map below.

For Those Coming by Train

From the exit of each station

Leave JR Bentencho Station through the ticket gate at the North Exit, then take the stairs on your immediate right.

Leave Osaka Metro Bentencho Station through the west ticket gate and head to Exit 2A.