Dtilt has a pretty large hitbox, larger than that of ftilt. Use it when it's not the right time to use zair, missiles, or charge shot. Jab>dtilt is fast and effective. It's also Samus' fastest launching attack. Launch your opponents into the air, fire a couple of homing missiles to force them to airdodge, then attack them as their airdodge ends with whatever aerial fits the situation. Dair is your next most effective launch. It begins to launch foes at around 30% depending on weight. Dair also has a lot of range, practice with it in training mode to see where you can hit your enemies with it. Follow up with utilt, uair, or screw attack. Samus has a solid aerial game, so focus on getting your foes into the air where your game is. Good luck and happy hunting.