I haven't looked at the Ghost Shooter tutorial myself--would it be possible to simply omit the parts that use the formulas? Either that or perhaps simply altering the tutorial .capx would suit your needs better.
Regardless, anything new takes some time to become proficient with. I'd suggest diving into some of the other tutorials, looking at this help forum, and simply playing around! Construct is great for that! If you're dead-set on making a game akin to Ghost Shooter, spend a little more time with the tutorial and see if it's possible to cut out the parts you aren't understanding. See what happens!