How to effectively counter double sprayer/metal shreddersers

For Double Sprayers, always use Steel Stampede. Physical Shotgun can be good too.

There is no best torso in order to counter Lava Sprayers. You're best bet is to have 100+ cooling, which is easy to achieve with the help of heat controls.

And finally, always not forget to put in a Saurus Rifle. Make sure to have high damage output such as the following;

Armor Breaker
Ultra Nova
Metal Shredder

That's about it. But in order to seal a win is to have critical decision making. I can't explain that one - You're on your own.

Now for Metal Shredders

You basically need an Ultra Nova/Armor Breaker in order to effectively counter it. The help of knockback items like a Firewall will be extremely helpful.

Bully can be nice if it's combined with Ultra Nova.

Double Metal Shredders are commonly used with God Modes that has -24 resistance. 10 Physical Resistance is more than enough to deal with Double Metal Shredders that uses God Mode due to God Mode's negative resistance.

Average HP of your Metal Shredder build in order to counter Double MS's is should be around 700 HP.

As always, critical thinking/decision making is a must-have in order to seal wins.

PS. Don't complain about not having this items and get destroyed by them - It's the current meta of SM, you don't have any of the items stated above, you will hardly win any battle.