The term "landing assured" is used in a couple of different ways. In the way you mention, with a helicopter, it's used in this way: Take PHX for example. Within the PHX tower's Class D airspace, there's numerous hospital helicopter pads located within. Helo's heading to these hospital pads must contact PHX tower, but only because they're entering the tower's Class D (apart from the overlying Class B airspace); but since PHX tower can't always see the exact helo pads, it's more of an advisory call by the helo in order to get winds/altimeter/WX, etc. This also requires tower to issue a "land at own risk" call. So the calls will normally go like this:
"PHX tower, AirEvac 9 is 5 north for St. Joes"
"AirEvac 9, PHX tower, PHX altimeter 29.96, winds 350 at 5, landing will be at your own risk, report landing assured"
And the helo will report when it's on the deck at the hospital.
The other place you hear "landing assured" is with fixed-wing on ASR/PAR approaches. On a PAR, the GCA controller, as he's talking you down, will advise you when you reach DH. Call will be "Hog 45, at decision height". To where my response will be either "Hog 45 landing assured", or "Hog 45, executing missed" ("executing climbout", for practice approaches). If I call landing assured, the next call from the GCA controller will be "Hog 45, contact tower on rollout." With an ASR approach, the controller will call when at MDA, and when at 1 mile from the runway. I would then make the same calls at the appropriate time.