Showing off my level 100 team beating the special Halloween event (which gives good equipment) as well as the second-last story mission. This team is in NG++ which increases the level cap (and therefore the difficulty due to enemy scaling) to 100.
In your first playthrough where the level cap is 30, you'll find that Heavy Armor is quite effective at soaking damage, so a Force Psyker, Soldier, or the Prototype are all useful at drawing hits. However, around level 40 or so, the armor you find doesn't scale up at the same rate that enemy damage does. It therefore becomes important to tweak your gear to focus on maximum Dodge % chances. Light Armor becomes top tier for all classes in NG+ and especially in NG++ where enemies can kill you in a couple of hits.
Smuggler, Force Psyker, Void Psyker, and Cyberninja are my favorite team. Smuggler with maxed out traps gets free attacks at the start of battle which is insanely useful, and their gambling tree has some of the fastest and most effective attacks in their arsenal. Slot Machine and Coin Toss's bonus damage both are undodgeable attacks, and if you're using a high powered weapon like a Rifle or Swords, they'll hit incredibly hard. I don't tend to use pistols with a Smuggler since other weapons allow Slot Machine to hit very hard with low time costs, but I decided to use a pistol for this run. It fit with the character art. ;)
Force Psyker's one of my fave classes. You get a ridiculous number of passive abilities including a once per battle autorevive for yourself, and a separate once per battle autorevive for any member of the team. You also get a mix of powerful stunning abilities, abilities that deny the enemy attacks, abilities that nullify status ailments or soak damage, and a summon you can charge that also acts as an autorevive effect for team members. Give 'em a powerful heavy weapon like a rocket launcher and all those low time cost skills can be pumped out very rapidly despite the rocket launcher's slow speed for an incredible amount of damage.
Void Psykers are insanely strong. You rip up shields while charging up for a big unleash. Single-target Torrent is great, but even better is Voidstorm, which randomly hits enemies for massive damage + chances at instant death. With near 100 void energy you're almost guaranteed to get two instant kill hits at least, if not killing by sheer damage. And Force Psykers are handy as backup because their revival abilities can help protect you if the Void Psyker accidentally blows themself up with void energy.
Cyberninja's got one of the best team buffs in the game, making your team nearly unhittable for several turns, and their Combination skill tree has all their best active skills, allowing you to get in hits with negligible time cost and building up for a massive damage Takedown that will instantly kill most bosses if properly charged up.
I don't use a Hacker much. Unlike Shadowrun, which this game is thematically based on, you really don't need a Hacker for disabling security and their combat skills simply aren't as powerful as other classes tend to be (though they're not useless by any means). I've run multiple teams up to level 100 starting on Ironman / Hardcore, and I think any combination of 4 classes is viable as long as your equipment keeps up with the enemies (align yourself with Tec to get autorevive items which can help avoid permadeath).
Note that in Ironman, if a character dies without an autorevive effect kicking in, you lose that character for that playthrough. However, when clear the game and start the next NG+ cycle, you can rerecruit any classes that got killed in the previous cycle (though you still lose their equipment they had!).
HALLOWEEN MISSION: To do this mission you have to set the Holiday in the options at the title screen to be Halloween. Then, find candy in random missions (they don't appear in story missions). You can cash in on various goodies, but the ticket costs 10 pieces of candy. Inside are multiple unique bosses with automatic summoning gimmicks, and the boss of the mission is an enemy with a unique status effect, Fear. Every time his turn comes up, he inflicts Fear on all members (even if he's stunned, he does this before he attacks). The only way to get Fear resistance is to equip the accessories you can buy from the Halloween vendor, by letting yourself get possessed by the souls you find (who lower your accuracy but raise your Fear resist) or by eating the candy you find in the mission.
Or, just use a Force Psyker with Purity Prism. The Purity Prism will block Fear as long as it's active, and if the Force Psyker somehow gets caught when Purity Prism isn't active, he has an auto-status cure passive that can kick in! The Prototype is also immune to Fear as are the Engineer's summoned turrets, so those work well too.