How do skills work?

I'm faced with a choice between two skills. How do they work? When can I use them? How long do they last? How much damage do they do? I want to know before making my choice in case one is clearly superior to the other. Are there any more skills?

Solution 1:

Both of these skills can be used during combat with a cooldown:

• Thunder Bolt deals a set amount of damage based on the skill level (starting at 20, +7 damage per level) with a cooldown of 20 seconds.
• Invulnerability makes you immune to all damage for a set amount of time based on the skill level (starting at 3 seconds, +3 seconds per level) with a cooldown of 30 seconds.

You can upgrade your skill after choosing it for a gold cost. Each upgrade costs 500*skillLevel*skillLevel+500 gold bars, with no ceiling on the skill level.

Solution 2:

I want to know before making my choice in case one is clearly superior to the other.

TL;DR: I find Invulnerability to be clearly superior to Thunder.

At the first level, Thunder is essentially an extra attack. Invulnerability protects you for three seconds at the first level, also effectively an extra attack as it allows you to attack once with no successful response. At the second level, Thunder gives 7 more damage, a third of an attack. Invulnerability prevents two attacks at the second level.

Beyond that, Invulnerability scales automatically with the difficulty of the monster that you face. Whether it does ten points of damage or a thousand, Invulnerability prevents every attack while it is running. Thunder does the same damage to every monster and can easily be outstripped by your sword. For example, my sword currently does 100 damage and I have a level 9 Invulnerability. So Thunder is a bit less than a single attack. Invulnerability now prevents about ten attacks and so allows me to make ten free attacks.

Many monsters can kill me in two hits, as they do more than half my health in damage. So with Thunder, I would do one Thunder and two regular attacks. With Invulnerability I can do skill level plus three regular attacks. Since regular attacks are more powerful than Thunder, adding just one regular attack is better than Thunder.

I have never successfully gone through a long enough fight to use Invulnerability a second time. Perhaps Thunder's shorter cooldown would allow me to use it a second time. But even so, two Thunder casts is less effective than two regular attacks.

It's possible that Thunder is a lot cheaper. But note that Invulnerability is a lot more effective. So Thunder would have to be something like ten times as cheap to catch up. And that's assuming that the next group of monsters isn't even worse, doing more damage than I have health.