So, my friends and I just beat adventure mode in Magicka. After defeat the most powerful wizard in the world (twice), a giant dragon, and Death himself, among others, we were expecting a fairly challenging final boss, to say the least. Instead, we just thunder bolted him a couple of times and beat him about maybe 20 seconds into our first try, before he was able to cast his first spell. So my question is: was the last boss in Magicka intentionally made easier than all the others as a final joke?
If you have 4 players who cooperate using drenching and high-damage electric attacks, Assatur is indeed pretty easy to kill. I don't think his easiness was intentional, nor is he that easy if you use non-electric attacks.
If you're looking for a challenge, try beating Assatur without getting him wet.
It could be just my perception, but Magicka was rebalanced a couple of times since its release to make it easier, but also to nerf some strategies deemed way too cheesy. At first, this game seemed to require a full team of 4 wizards to beat it, still failing multiple times to return at the beginning of the chapter. After the many changes, it seems to be balanced for one single wizard of kinda low skill.
Same goes for the bosses: they feel a lot easier now. Yes, none of them are a real challenge in a powerful team. However, some still are for one player.
Also, note that Magicka is a game of counters, each boss still has a very cheesy way to beat him. For example, two charged ice balls (QR-QR-QR-QR-D, hold RMB before releasing) always kill the Jormungandr, and you have lots of time to use it. If you intentionally avoid (or don't know about) such a strategy, every boss becomes significantly harder.
If you are looking for a real challenge, sharpen your teeth on the basic Adventure campaign, and then try things like The Stars Are left, Dungeons&Daemons, Dungeons&Gargoyles. The latter two also have difficulty switches at the beginning if the game is still too easy for you, use Google if you want to learn how to activate the switch (too hard to explain via text).
You can also try Challenges, some are easy (the Vietnam Survival one), some are very hard, e.g. the Havindr Arena.