If your 50 Kael already can do 12-6 on brutal, you've done a good job so far. Depends on his time to clear the stage to make him or another of your champs 60. You can use Kael in many other places in the game, so you're doing nothing wrong with him, even if you've got Banshee as a better poisoner. As a farmer Bellower is much better. Kael is way better at clanboss which needs other artifacts than a single farmer needs unless you've got a uk team.
Check your artifacts. If you have some more lifesteal artifacts and can speed up Bellower to approx. 160 and will get him to almost 100 kritq and perhaps 150 (better more) kritd you could try Bellower. Perhaps that would be good enough for 30 to 45 second runs, don't know. I was really long farming stage 12-3 with Elhain which really was chewy (1 minute plus). After some months or so I've got Bellower (atm at spd 221, kritq 100, kritd 238, atk 3000, no lifesteal, helmsmasher mastery) who does it much faster, though the last wave always needs two hits (better was 250+ kritd as already mentioned above by another writer). Hopefully that helps.