The Marine is useful for stunning bosses and knocking them out quickly with his laser. He also has grenades and units that can help deal with crowds of creeps.
The Rocketeer is good for dealing out damage to large crowds of enemies, and his nuke is great for saving walls that are failing.
The Tinker is a support hero. He has a move that heals structures/units and a move that makes structures build instantly. While in his tank form, he gains a thorns aura.
The Commander is also a support hero. He has an aura that boosts the damage of nearby units, paratroopers that can be summoned through the game, and a healing move that's great for keeping other heroes alive.
The Sniper is the best hero for single target damage later in the game. His Frenzy, Critical Strike, and Metamorphosis result in huge amounts of damage when maxed out.
The astronomer is a hero with great AoE damage early in the game. The longer the game goes on however, the less effective these skills will be.
So, if you want to counter bosses, use the marine.
To counter groups, you want the rocketeer.
To support a base, use the tinker.
To support other units, use the commander.
If you want something that deals a lot of damage in the late game, use the sniper.
If you want something that deals a lot of damage in the early game, use the astronomer.
With the items/buildings available in the game, it's possible to beat any difficulty/mode with any hero. It's all a matter of finding out what works best for your style.
