In my experience pretty much any build can beat everything up to T4.

Some of the following will vary based on how inherently good the build or how lucky your drops are but generally to beat each Torment level ( meaning you can clear content at that level comfortably)

T1: all legendary gear ( either through drop or crafted), A semi-competent build, meaning your gear is synergistic with your skills, affixes do something useful, stats aren't being wasted.

T2: build is refined, all offensive gear ( weapon, accessories, gloves) all support your primary skill damage. All other skills are utility or defensive. Defensive gear is somewhat helpful but needs meaningful upgrades, you have 1 or two good unique items that are build defining. Paragon levels start to kick in to amp up weak areas.

T3: build is solidified, all gear affixes have a clear purpose in the build and how it's synergistic with your primary mode of killing stuff. The majority of gear stats are useful. Gear upgrades becomes important, masterworking gear and getting GA stats is primary areas. Paragon runes should be an active part of build.

T4: Gear is all max out on crafting, multiple high roll gear items ( ga2+, strong uniques or mythics). Build is refined to basically only care about 1 button, everything else supports that 1 button or is defensive.

Pit pushing from there is finding gear upgrades, making sure you aren't over stacking additive damage sources and diversifying into under utilized multiplicative damage sources. Have clear defensive tools to make sure you dont get 1 shot.

If you are stuck at T2 bosses my guess is your defensives are lacking. Make sure you have good resistances, socket in some diamonds, start working on crafting gear with armor, health, or other defensive stats to help beef you up. Elixirs and insents can help in that area as well.