Hmmh... It is difficult to get torpedo hits on a target, who is aware of your presence to be sure. BUT it is doable. There are several tactics.
1) The Island Ambush. Try to wait and place your torps so that the victim will be passing close behind an island, when they meet up with him. Use narrow spread and place both tubefulls into the white aiming indicator range. He will not have the time to dodge them.
2) Unwary BB going full tilt with their side pointed at you. Use 2 overlapping spreads. I usually fire first 1 spread which is about 50% overlapping with the white aiming indicator, but slightly to the back of it and then fire the other right after similarly but aiming slightly to the front (again 50% overlap with the aiming indicator) so the middle torps in both spreads will interlock. This makes it more difficult to dodge them. If you have 3rd launcher, you can either keep it in reserve or expend it to fire a 3rd spread either in wide spread or a narrow one right into the middle of the previous 2 spreads or alternatively at some other target entirely.
3) You have confirmed it is an experienced player and has dodged your torps already. This is the most difficult. I usually time my launches on set intervals (I call this a "Torpedo Skirmish Mode"). For Fujin/Minekaze/Kamikaze it should be some 12-14 sec apart so you will launch them at regular intervals. But not according to indicator, cause he is now actively dodging. Observe his weave pattern and see how he turns, then launch 1 launcher at a time to his estimated heading after the next turn. keep this up and some will hit. This tactic relies solely on torpedo saturation for success, you simply launch them in intervals in hopes that eventually the target will fail to anticipate your launches and during his lapse you will score a few hits (rarely works on Cruisers so do not waste your time if you can avoid it, but it does on BB, due to their slower rudder shift).
OR choose the easy tactic and just change targets.
By and large torpedo is an ambush weapon, so once you are sure your target has been alerted, just leave it for now and look for some easier pickings, as a wary opponent will be able to dodge them quite easily. Also make sure there are no other ships on the way, who might be able to spot your torps prematurely. It also pays to try and ambush ships, which move in "flocks" - 2 or 3 ships together, which are too close to each other will hamper each other's ability to dodge torps and so you will be able to get more hits.
Oh yeah, the best way to take down this kind of opponent is to wait until he is engaged dueling with some of your own team members, gets distracted, tunnel-visions and perhaps forgets about you, if he tries to bow tank them, go to his side and launch torps, if he dodges them he will be showing his flank and eats a citadel from your allies, if not then you'll do him in. It will be a win either way.
Also it is always best to launch your torps against a target, which is heading slightly TOWARDS you. If they are heading away from you, save the torps, because they will likely run out of range before reaching the target and anyway the spread will be so wide that they are going to be easy to dodge. Also - For this same reason, do not waste your time trying to launch torps from maximum range, always try to sneak as close as you can while making sure you are not detected, of course. This ensures minimum spread and maximum effect. If target is showing its side, but slightly angled away from you, it is best to use your speed and re-position yourself slightly ahead of him before attack.
But most important of all. BE PATIENT! (more details for the geeky included below - Yeah, i have too much time on my hands)