I have about a 25% winrate in onslaught going completely solo playing a variety of champions. Currently I have yet to beat it as an ADC since that is by far my worst role but I have come very close (Got the final boss down to about 10% health) We got swamped by the number of enemies after Ahri went down and no one was able to get over to her though.
It is a hard mode but it is most certainly doable, and this is as a Gold player. Most people I queue with don't even have borders either. Rek'sais aren't tanky until very late (Or never if you have a jinx + lulu combo. As Jinx with Botrk, rapidfire, and inf edge I was killing them in roughly 3-4 seconds, cho's in about 6-7) and Cho's are pretty much irrelevant so long as you make sure to take out any rogue skarners who pull you into them.
The only map cho's are actually annoying on is when you have to stay in the circle on top of that small bridge, but only 2 of them come then and there are few other enemies so they can usually be wiped pretty quickly.
Coordination does not require 5 man teams. I have discussed strategy during the shopping breaks and acted on those strategies with 4 people I have never played with before, and I do it pretty consistently. Also while the overall mode is definitely hard that stage is easy if you go with that one tactic.
I am also going to add into my post that after more games Jinx severely outclasses MF as your damage dealer. As a bad ADC player I was outputting roughly 75% of the team's damage with Lulu + Soraka keeping me going. If there was a Janna shield on that the damage would be even higher. She also has range which is nice. Ideally a Jinx + MF + Janna/Poppy + Lulu + Soraka team seems to be the best, because MF ult is great for clutch situations where enemies get out of control. I also found that the mode is definitely beatable without a tank, though the Vel levels become much harder unless your whole team is on point with their dodging.