1. About Twin Tailed Comet and Burn It Down. Lets say I have 4 developments in Battlefield and my opponent only 1.I play Burn It Down ("Action: Destroy one target support card with printed cost X or lower. X is the number of developments in your battlefield") and in response he plays Twin Tailed Comet, as I understand he may destroy one support with cost at most 1, right?

2. May I use Grudge Thrower action multiple times in my turn if able - having enough units and resources (i.e. 3 times after defenders has been declared by my opponent in battlefield phase)?

It copies the effect, the effect is set at X, X is defined as the number of developments the player has, he has one development, his upper limit cost is 1.

If he complains tell him he should be happy that X is not part of the cost, because since Twin-Tailed Comet specifies without paying the cost, X is always 0.

concerning the point 2:

the answer is yes.

Can all actions be used multiple turns if you have enough resources and the card doesn't say one use per turn?

It's quite a hard thing to get used to after playing Magic if it is the case.

Sebly said:

Not sure I understand the difficulty : isn't that how Magic plays (with Mana being the local name of Resources) ?

Thanks for answers. About actions, those which can be used only once per turn has additional text "limit once per turn" like Bright Wizard Apprentice .

It's just that in Magic usually once you use an ability or attack you have to "tap" the card and it can't be used again until your next turn. As far as I can tell in WI, you can use abilities more than once per turn?

That is correct. Most of the abilities will come with another cost which is the real limiting factor, or are only triggerable under specific circumstances though most of those can be triggered every time that event takes place.