I haven't gotten to Veleno in heroic mode yet, but I remember it from storyline. Massively nasty AoE, he gets it off, you loose. Which is pretty much the same as Widow Queen. Given that, I wouldn't expect her to be very useful against Veleno. Except maybe if you have a tank tough enough to take its normal damage and someone like Calvin around to get rid of Veleno's morale. Not sure what you were expecting Maria to do, especially considering she has a very high miss rate.