To be honest with you, if you aim to hit big scores in survival mode, you'll need to have a good character who is at least level 20/25, and you will need to finish the main storyline to unlock all of the power-ups. You can't hit 1M without any powerups unlocked or with a level 1 character. (Unless you're a pro)

So let's begin!

So... You want me to have a level 25 character before trying out survival mode and hit the leaderboard? But what do I upgrade?

Simple!
If your character is a soldier (I can only give my opinion on the soldier since he is my main class), you need to upgrade his :
1st. Life
2nd. Speed
3rd. Regeneration

(Your priority must be his HP and Speed)
Upgrading regeneration is good aswell as upgrading his shield. It can save you in many situation.

----------------------------------------------------------------------------------------------------

Concerning the abilities; I think the flame turret is the best offensive spell and the plasma shield is definitely the best defensive spell.

The flame turret is good to use when you feel like you're running out of ammo on your gatling/flamethrower and that you have only a little riffle to kill annoying ennemies like the zombified versions of the heroes.

The plasma shield is a must-have, it can save you in many situation. When you're being cornered just activate it. It reduces the damage a lot, like 90% maybe. Another use : if there is a lot of ennemies and you have a rocket launcher, you can also activate it, dive in the swarm and then press right click to kill all of the little aliens. This will allow you to get lot of power-ups and medkit. (and to reduce the number of ennemies which will allow you to focus the bigger one)

-----------------------------------------------------------------------------------------------------

As for the perks :

Must have
- Rhino (must have IMO, can save you, if you get cornered near wall)
- Vampire / Pureblood (killing ennemies make you regain hp, do I have to explain why it is important? )
- Regen 1/2/3... (Help you regenerate so it's good)

There is many others perks which are great to have like minimap, recycle, hyper senses... But they are not "must have" so don't worry if you don't have them.
Anyway once you unlock fire turret & plasma shield, always get the perk while leveling up. All perks are good to take.

------------------------------------------------------------------------------------------------------

And you? What is your build?

If you're curious :

So, once you have your character level 25ish, all of the weapons unlocked, you need to know how to proceed, which ennemy to focus, which weapons are the best...

No worry I'm here to help you!

The beginning of the survival mode in any map is pretty easy. You'll get only scorpions and basic aliens/zombies until 60k ~.
However the first problem when you hit that 60k, are the "zombies heroes". They can make pretty good damage especially the engineer. They are your first priority along with the dogs throughout the game. Even at 1M or more, you need to take them down quick, if they are nearby you. Your second focus must be the bosses of course. But to take them down you need pretty good weapons.

-------------------------------------------------------------------------------------------------

Your best bet is the gatling or the flamethrower. I personally prefer the gatling gun, but it's up to you. It is one of the best weapon in the game and you can do MASSIVE damages on all of the ennemies. Another good thing to pick up while you have a gatling is the plasma riffle. Once you run out of ammo from your gatling or flamethrower, it is definitely the second best gun in the game. It can two-shot/three-shot a quadruped boss if you have the energy ammo upgrade perk. (which is awesome).
The best choice in term of secondary weapons, is without doubt the ion beam. It is always good to save a ion beam for a boss, or when they are grouped up. Once you used your beam, I think the good ol' Rocket/Grenade launcher or AC130 are your best choices.

-------------------------------------------------------------------------------------------------

As for the powerups, once you see one, just run over it. There is no "bad" power up. The only thing you need to know about them is that the shield is op tier, and that "slow time" gives you back full ammo in your current gun. (So use that at your advantage)
It can be useful when you're running out of gatling ammo for example.

------------------------------------------------------------------------------------------------

At some point (2M + ), you'll need to use a technique "Hit & Run". You'll have to regroup the ennemies, spam your left and right click at them, recharge your plasma shield, and then once you can use it, press the button and go into the crowd, and use the grenade launcher/rocket launcher at your own feet. Once your shield run out, run away and recharge it again... This is the only "viable" technique I found to do massive points while surviving.

Example :

There is no secret, you need to practise your aim and your map awareness. I can only give you tips to help you! But if you tryhard you're going to make big score! Believe! :D

I hope my guide helped you to get the survival achievements! Have a nice day!