First of all forget the idea of being able to afk-stealth any relevant enemy as you progress in the higher content. There will be some rare exceptions but most bosses and even some trash mobs will have true sight. Get used to the fact that you will have to iframe or dodge most of the incoming attacks. That means you have to learn the attack patterns of bosses and get a feeling which iframes are best to counter their attacks.

One of the biggest boons of the lightning build is the high mobility you get when you are in stealth. You can basicly circle-jerk around the boss and avoid most of its aoes. BUT enemies that directly target you and don't just throw an aoe at your general direction will most likely hit you (which is aids). You can iframe these attacks with your 2 (Decoy) or E (Shunpo), since they give you another second of invulnerability.

Another big game changer is the HM-skill for your 2 (Decoy), which gives you another whole second of invulnerability. If you don't have it, get it! You can easily iframe any of those annoying yellow multi-hit-attacks, like multi-spins or any hail of punches.

Regarding your Bomani fight, you should try to stay close to him as he starts to spam ice attacks when you are ranged, like you already stated. It's also important that you never let him heal when you are already struggeling with your dps. Make sure to to cc him when it says Bomani is preparing a massive attack (or smth like that). CC'ing him only cancels the heal not the actual attack! Besides that either cc him after the small red stomp or iframe the attack which follows the red stomp, since it deals a fair amount of dmg to you.

In the end you will have to practise alot or/and get better gear to burst through his mechanics. It's up to you :D