Let's talk about my top Three League starters builds for POE 3.20 The Forbidden Sanctum. The following builds will be in no particular order, I think all of these are able to clear the content that is required for a league starter, which for me is everything up to Uber bosses. When it comes to Uber bosses, it's a lot about how good you are at boss mechanics, so it's pretty hard to draw a distinction there. Something I want to mention before we start is that the most important thing when it comes to picking a build, especially for your league start, having a good entry into the league is pick something that you actually like, even if there are some meta builds better than yours.
Then whatever you want to play, test some stuffs at the game, that's how you get experience. POE 3.20 Expansion The Forbidden Sanctum really isn't that competitive, unless you're a racer, so just have fun, just explore this Sandbox game.
Let's get into the list, number one is Righteous Fire, now this build is just incredibly fun to play, and it's also incredibly easy to gear, do not need too much currency. Righteous Fire is an area damage over time effect around you, and you're basically just shield charging around, killing and exploding enemies, while also having something like a fire trap against single Target. This is also a very hardcore but friendly build, very tanky with good regeneration.
As for downsides I guess I would say the single Target damage is not up to par with some of the other Builds on this list, but usually that doesn't really matter, because your enemies can not kill you. Most of the time you can just face the bosses and take chill, as this is one of the longest standing and probably safest builds that you can use in league start.
Corrupting Fever Champion
Then we have the Corrupting Fever Champion, Corrupting Fever is a duration buff that basically allows your attacks to apply a physical damage over time, which means you can use long range attacks like Kinetic Blast to apply those thoughts and completely explode whole screens. This is probably the build with the best clear speed on the whole list, it's just absolutely insane, you can annihilate whole maps. It feels like a end game build with the investment of a early game build, less POE currency orbs for an amazing result.
Champion is incredibly tanky on top, however it's not without downside. Your single Target damage overall is pretty weak, but consider how good it of a map clearing, it's still pretty strong, you'll be a lot faster through Maps than some of the other Builds on this list.
Explosive Arrow Ballista Elementalist
Then the next one we have Explosive Arrow Ballista Elementalist, Explosive Arrow is a bow projectile skill that whenever it hits an enemy will stick a fuse into it, can stack up to 20 and after a duration of one sec second there is an explosion, which is bigger and stronger depending on how many fuses you were able to stick into that enemy. This is a build I've played over several leagues and I can say it is pretty remarkable, it's extremely solo self friendly, because it doesn't require any gear. Yes, it basically has huge damage without needing any uniques or any specific crafts, therefore it's also a very new player friendly. On top of that, you're also quite easily get defensive auras into your build, like Grace, Determination ,100% Spell Suppression, so on that end it is also pretty good.
As downsides, the recovery of the build is a little sketchy, you will mostly use life flasks. As well as that some people just do not really like the ballista playstyle, as there can be delayed damage and some people just like to deal the damage themselves.
If you want to play this build on Hardcore I would probably recommend the Champion version, on Hardcore you really don't want to take your chances, instead you want all the defenses that you can get. Overall an extremely fun build especially with the huge Ignite Proliferations that you'll be able to get without big number of currency.
Poison Blade Vortex Occultist
Next one we have the Poison Blade Vortex Occultist. This spell creates ethereal blades that spin around you whenever you activate it, and it can stack up to 10 times increasing its damage with every stack. Basically the playstyle that you have is sometimes pressing Blade Vortex, and then you just charge around and explode screens. It can make for huge explosions, and basically not really having to press any buttons which is probably the biggest upside of this build, all you really have to do is pressing the Blade Vortex button here or there, and destroy everything rush from maps.
The downsides of this build can be a little bit squishy. If not built into it correctly in endgame, it would not perform well as you wish, but this is not really a problem anymore once you have your gear, just before that it can be a little annoying, also it doesn't have the best boss damage. But then again, it is poison, so it has a lot of solutions to that. I think this build is especially good if you want a build that's solid and without tons of POE 3.20 currency orbs early. You can also scale later, and you don't have to reroll immediately.