but that's not intuitive ... having to pick two stuff at the same time
You need to set the player's goal to one if you want your game to be fair with a time limit ...
The weapons drop out of enemies, so they won't appear completely out of your way.
It would be cool if you had a combo system which built up your "blast." The more enemies you kill in series, the bigger your death will be. You could build up tremendously huge explosions so dying vaporizes all the enemies on screen, adding to your final score.
I'm thinking about something along those lines. Not sure if combos are the way to go, but I want some way to increase (maybe temporarily) the final explosion size. Maybe a random power-up that drops out of enemies and increase your explosion for the next... 20 seconds or something. So you can either make use of it in those 20 seconds or try to survive longer.
Other things that would be cool are chain attacks; enemies give off a certain sized explosion so they can damage/destroy nearby enemies. A smart player could herd the enemies into a string and blast them all away.
There will be specific enemy types that will explode, the larger ones
You could also add spikes on the walls so platforming becomes more challenging.
There is an 'easy' and 'medium' difficulty level, I think this a good idea for the hard one when I make it!
It would be better if killing enemies added time, adding to the incentive to actually kill enemies. Maybe make the amount of time given reliant on the size of the enemies. You may spend extra time killing a big enemy but the time might be worth it. Killing smaller enemies in small numbers may not be worth it because they give you less time. It adds a more strategic element to the combat.
I'm afraid that if enemies added time there would be little incentive to go after the coin pickups and people would just stand in one place and shoot everything that comes close. Right now you have to constantly be aiming for the next pickup but also shooting enemies along the way to clear your path and earn points, because the amount of enemies killed is your final score.