All bosses except Abadede are easy in any difficulty, with a cheap repetitive but effective move, with any character. Also works for normal enemies.
The only difficulty can be punks or other bosses around, but most of the time the AI seems to have some problem there. When you're doing grab attacks, enemies around won't get close, they keep just watching, so you have space to do whatever you want. That works for two shivas event as well.
I remember doing it on mona&lisa boss, I took one at a time and the other was just watching around.
On those double bosses you can also try to keep one of them out of the screen, and attack the other close to the edge. The one that is out will take a while to get inside again because you're on the way (their AI is really very defensive). But be ware, the boss from outside can escape the trap and attack without a warning
Now the cheap trick:
Grab it. If you have any difficulty to grab some boss, hit first, then move to him/her (run if needed). Now it's the old cheap trick: do your grab attacks, vault, hit and grab again.
Do it fast for bosses like Barbon and sor2 bots, cose they can escape from your grab if you take too long.
I use shiva cose of his fast 3 grab punches. You can also try the old sor1 way: grab, knee, ungrab and grab again, but it's trickier and harder for some faster bosses (at least I wasn't able to time it correctly yet).
This works for all bosses except Abadede (so, this includes Shivas, Mr.X, Rudra, etc).
I used this trick to easily get gold medals on boss rush mode.
Edit: Robot Y is quite hard to grab, for that this trick may not work.
To grab a Jet, get close to him so he will rise up then charge on you (make sure to move away when he's rising up). then run for him, you'll be able to grab him.