There are a few things that I believe you need to do so that you can finish a level (unless you have the best weapons)
1. When a zombie horde is coming at you, you need to head for the exit even if you have not collected all the bonus crates.
2. At the very least, you need a medikit and defibulators (have two if you have a survivor with you). You may need to have one beacon grenade to clear the area when you need to get out of the level.
3. If you only have one defibulator and you and the survivor are low on life, just restart the level. You need one defibulator to revive one person..Well, sometimes both Evelyn and the survivor gets revived but in rare cases.
4. In a Veteran or Expert Mission, expect multiple special zombies at the same time. You may need to use better weapons or damage boosts to kill them quickly. Or get ready to use medikits and defibulators.
Thing here is that I do not want to buy gold just so that I can get the defibulators. But the good thing about the game is that without the best weapons you can still finish the missions.
Comments and feedback welcome.
The Wolf Pack are my friends, like all others whom I have the pleasure to know. You have my respect because you deserve it - the same way I expect this respect from you as well.