Those points are listed in some kind of order of priority from the simplest ones and what people reported as the most effective:

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1 - Use Windowed Mode with lowest graphics and no V-Sync
2 - Don't Alt-Tab or minimize window
3 - Clean PC's registry/temporary files and close background tasks
4 - Restart your PC and router
5 - Restart whole game between each chapters/levels or after a while
6 - Avoid level restarts so to not increase crash probability
7 - Play version 1.5.1.0 or 1.4.16.0 or use the Magicka Fixes on the latest version (1.10.4.2)
8 - Disable unused DLCs
9 - Keep backups of all freshly downloaded game versions to reset game folder occasionally
10 - Choose map after loading a mod, only when all players are in lobby; start only if all ready
11 - Player with best PC/internet should host; more guests is crashier
12 - All players should restart their game if one of them crashes while in lobby
13 - Check if game stopped in Steam before re-launching from external shortcuts
14 - Play in Win7 or in compatibility mode because game is crashier the more recent the OS is
15 - Don't spam "Spray of Judgement" Magick

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1.4 works differently than 1.5 or any modified versions of Magicka.

You have to move the real Magicka folder elsewhere and rename the 1.4 folder "Magicka" just like if it was the real one, and launch it manually with its "Magicka.exe".
When you're done playing, rename the folder back to "Magicka 1.4" and put the real Magicka folder back in the Steam folder.

It's to trick Steam into believing that 1.4 is the latest version (1.10) while you play. If you don't do any of this, Steam will update the files and your 1.4 folder will be corrupted.

1.5 doesn't require this, you can give any name to the folder and keep it side-by-side with all the other versions as if they were all entirely different games in your Steam folder.
Launch them manually, and there will be no problem.
It's the same as with 1.5 for any moddified version of the core game, such as Magicka Ciepicalypse, MagickaX, Magicka++ and Magicka Ultra Hardmode EX.

Also, never use the "Magicka Fixes" community patch on the older versions. It will corrupt the files.

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I hope this helps.

For further assistance, don't be shy to ask us in the Troubleshooting Channel of The Magicka Camp.

The Magicka Camp:
https://discord.com/invite/xPR8JmxwUS

Magicka 1.5.1.0
https://drive.google.com/file/d/1xdxmPLnMdZZejgLdTHFEoMl4_lVKfKmW/view

Magicka 1.4.16.0
https://drive.google.com/file/d/1uA-Gbb_SoEGp6-DysokahN-NjmqVhLhd/view

Magicka Fixes for 1.10.4.2
https://github.com/pj1234678/MagickaFix/releases/tag/v1.4

Reddit version of this post
https://www.reddit.com/r/magicka/comments/1bykpzh/magicka_crashglitch_prevention/