How does progression work?

[Andrew R]

I keep dying around wave 17 first difficulty tier.

Than does another open up, and another? How many bosses are there? When Is the game considered "beaten?"

[Lod101]

20 is the final wave.
and for unlocks look at the achievments.

[R3sistance]

There are 5 danger levels, one boss until danger 5, but you gets more elites and hordes appear and things get generally stronger faster the higher danger goes. Achievements are for beating game with every character and beating every danger level, the game tracks which characters have beaten which danger level (indicated by background colour) but 100% only needs you to beat danger 5 with one character. Each time a character gets their first clear, they unlock a new item.

[r0m4n0]

achievement = unlock new character
complete a run, boss included = unlock new weapon or tool

[Andrew R]

So what it boils down to is I'm in Level 0 right now. Complete that, move to 1 and 2 and so on?

[Cryten]

difficulty levels do not matter for unlocks outside of the 1 time unlock of beating each difficulty level. IE you only have to beat difficulty 5 once with any 1 character to get the difficulty 5 unlock. Otherwise all other unlocks can be done at difficulty 1.

As for you dying, concentrate on buy just 1 weapon, get 6 copies of the weapon you chose before fusing weapons for hiring levels. Always keep 6 copies for the set bonus having 6 weapons of 1 type. Always keep moving, basic circular patterns will work well. Sub machine gun, sticks, spears and shotguns make for a good weapon choice for a starter in the game. Shotguns in particular work for newbie because of the natural pierce.

It is wise to keep some speed, aim for at least 14-20% by wave 14.