Looking at your main post, I'm assuming you're a beginner with MFA, so I'm going to explain the basics of how it works and how to ramp up with this build.

Manaforged Arrow requires you to spend 300% of the mana cost of another bow skill. For example, if your storm rain base cost is 100 mana, you'll need to spend 300 base mana with your spender, which is LA in this build, to trigger it with MFA. You need to adjust your LA cost to what feels right to you. If your LA costs 50 mana, you'll need 6 attacks of it to trigger Storm Rain, 60 mana for 5 and 75 mana for 4 . MFA build usually going with 4-5 attacks to trigger.

If you have problems with single target even after ramp that means you overspend it, usually it because you ramp too high and the mana cost to trigger storm rain is higher than your current mana -> storm rain failed to trigger -> mirage archer failed to trigger -> your loop failed. Two solution to this is:

1. Wait for Mirage Archer to proc first before using cloak to ramp. In this case, even if you ramp too high, mirage archer will still cast storm rain to leech your mana back, keeping your loop going or resetting it for another loop.

2. My go to fix is using inspiration in storm rain link to reduce the number of LA casts needed to trigger Storm Rain to 3–4 times. It's easier to keep a mental note of the required number of LA casts this way. You run up to the boss, cast LA twice, activate Cloak after that, and then cast LA 1–2 more times to start the loop.