A few of them have some very specific timings to get right, but it helps understand how each one works, since when you use them it's really obscure how they actually hit people.
First thing to know is that your core's energy conductivity affects the charge up speed, and your generator's critical power affects how much you have to charge up in order to use them (there's also a modifier for each weapon). So high critical power + high conductivity = more time to use the weapon, less time charging up
Grind blade actually has a large hitbox in front of you and dashes your character across the ground when you use it, so it's best if you "angle" it toward an enemy and actually DON'T use it right in front of them, but a little distance back so that they are hit by the dash. It's a circular hitbox in front of you, basically.
Mass Blade has a horizontal sweeping attack in front and behind you, it's not that hard to hit with but it has very little range.
Giga cannon is self explanatory, but it's easiest to not actually aim for the enemy, but to hit them with the splash damage from the explosion, by shooting a wall near them.
Legion pulse is also self explanatory, but the range is a lot shorter than it seems, you do need to get right in their face when using it.
Giga Missile actually has an FCS lock on, so you can use target guns and/or fast locking missile FCS to make sure it hits. It's probably the best ultimate weapon in the game (you can easily land 2 hits with it) with the caveat that it makes you extremely vulnerable while using.
Mega Blade is the second best. It's a blade similar to grind blade, but instead of a forward dash you to an aerial lunge with a diagonal swing. It has an EXTEMELY long range, but a very narrow, precise hitbox, so the margin of error is very tight. It also has a lock on feature for where the lunge is directed, just like laser blades The main problem is that because it swings on a diagonal axis, it's very easy to miss with.