I strongly recommend dispersion shields on the lychguard and to consider bumping the unit up to 10 at some point. They are an extremely capable, hard hitting and resilient unit that can benefit from my will be done, lords will and resurrection orb as they are core. If you do decide to go with the staves though then they should be your primary target for the chronomancer's ability, otherwise use it on the skorpekhs who are hard hitting but surprisingly fragile.
I like reapers on the warriors, a blob of 20 puts out a lot of hurt. Due to their short range they really do need a delivery system and some support though - I like a Royal Warden with Veil of Darkness relic. He can teleport the whole unit into range for a punishing alpha strike and then pull them out of combat whenever they get engaged.
The Nightbringer is an extremely powerful unit and potentially the star of your army. He is also by far your best way of dealing with Marine characters rampaging through your lines - something which you are otherwise hard pressed to counter. He does require very careful use though - stat wise he is extremely fragile and most of the nightbringer's resilience comes from the cap of no more than 3 wounds *per phase*. You need to position carefully to minimise the phases he can take damage in. Charge his target with something else first to avoid taking overwatch damage in the charge phase (and also clean up any pesky thunder hammers that might be left after he attacks) and position models around him to soak up smites in the psychic phase. Use obscuring cover where possible. Also watch out as some stuff can deal damage in the movement phase - one example is that stuff you kill with your C'tan powers (which happen during the movement phase) might decide to shoot you in the face with a melta weapon under the effect of a nearby Ancient. You regain 1 wound per command phase, 2 with one of the protocols, if you can take damage in only 1 phase per turn you will get many turns of stomping on your enemies army. There is a fine line between the nightbringer being an unstoppable killing machine and a very expensive addition to your dead pile.
Use your nightbringer, lychguard and skorpekhs + lord as counter charge units. Use the warriors and scarabs to take the first hit. Use 5 strong immortal units and deathmarks for sitting on objectives giving fire support. The Skorpekh lord is something I recently started looking at as a hard hitting combat unit that can use look out sir and the lychguard's bodyguard ability to avoid incoming fire, something which the normal skorpekhs are very vulnerable to.
Finally I'd tend to recommend using the ark as a ghost ark. The doomsday ark just got a points drop and is actually very viable compared with the doomstalker but my reasoning is mostly practical if you ever intend to paint it - assembling and painting the ark model will give you a new understanding of pain and suffering. It has a massive surface area for cleaning mold lines off. It also really needs to be painted before full assembly - I got it down to 9 separate parts by gluing the passengers and guns onto each rib section, then painting the front and back halves as a separate batch due to the size and gluing the 9 pieces together after painting. Given any sane person will never want to do this again make the first a ghost ark, which you only want 1 of and does something unique, and then use doomstalkers for long range fire support as they are a push to fit model that is fairly easy to paint.