Alrighty so i was playing janna and i had a thought as i chased the crab down the river about 4 times. Why not make a trinket upgrade or an item that allows supports to do more dmg to that thing. It has its uses, opens up faster ganks to mid for the support or jungler.
Upgraded 
with new crabhammer passive.
You could bring smite. It synergizes really well with Targon's too.
rylai's will make it easier to kill the rift scuttler. but it isn't really supposed to be something for the support to kill solo. it's for the jungler, although i'm sure he wouldn't mind help with it especially if you have cc. maybe ping it when he's nearby and you guys can get a little river control.
I don't even bother with the Crab anymore I just plonk a ward and go on, even as jungler. It is tankier than any jungle camp, it takes too long to kill and the shrine stays too short to matter.
Trinket would be interesting, or just making it less tanky so that non-ADCs have a chance of killing it in less than 20 sec.
