Two shotgun and a melee can also kill a Bubble Titan. I think a pinpoint headshot and melee also kills. Might be possible to one two punch melee. Bubble doesn't really grant much hp +300 if you leave the bubble and have helm 60 hp. Bubble just in general isn't going to help much even in 3v3's in Trials it's pretty easy to counter especially if you're a Titan since one single barricade can kill a Titan's overshield