Theyre is a Negator which stops all magick attacks, you can take it out with Wakkas attack to get LuLu to dish some speels out, But your better letting the negator living, because the machina boss will let loose a huge Mana Beam when its gone
Leave the negator there, get out Auron, LuLu and another strong physical hitter, have LuLu use reflex every move (to help dodge the gun attack) and make Auron and whoever else dish some pain in, Hastega and Slow is also useful.