CC could be a little more usable on eximus in general, I wish they got the Sentient/boss nerf to CC abilities where the abilities just have diminished effectiveness rather than it either works or it doesn't.

I've been running around with a resonance Banshee a lot for this event, it's nice seeing lots of highlighted enemies spreading the debuff around. I don't think Silence makes that much of a difference when you can out walk the beams of death, their damage still seems bugged to me though, Covenant shows almost 2M damage prevented if I stand still for the duration of one of the beams. I don't think it can damage objectives though.

Overall, I think the game mode is pretty fun/relaxed. It doesn't feel like there's a meta loadout being enforced, and for the most part, you have the freedom to bring whatever you want to the mission.
There doesn't really seem to be any failure condition either (other than maybe the first objective, but I've never managed to fail there).
The pity system is also nice to have for anyone who doesn't have the time to farm or has terrible luck and the event cosmetics also look really nice.

Hildryn is one of the more fun tank frames to play, and is actually pretty good for harder content if you're having trouble staying alive.

Balefire unfortunately falls off in higher level content, exalted weapons are probably in need of an update to make them on par with current weapons.

Pillage is a very versatile ability, removing armor/shields from enemies and replenishing the players shields and cleansing status on the caster and their allies. As long as you can keep Hildryn's overshields active with this ability, toxin damage will not be able to bypass your shields.

Haven along with the Blazing Pillage augment makes Hildryn usable against the infested too while providing a bit of extra CC with the Heat procs.

Aegis Storm is also kind of useless, it reduces your mobility and only lets you use Balefire as a weapon. It can generate energy orbs but those are really only useful for your team mates and only moderately useful to Hildryn.

Rakta Dark Dagger is a pretty good weapon to pair with Hildryn as an alternative to Blazing Pillage or if you just want to quickly max your shield without waiting for Pillage to end.

I also like using Chroma's Elemental Ward (Electric) and replacing Hildryn's 4. It both gives Hildryn even more shields and reflects damage done to Hildryns shields back to enemies. Abilities that drain Hildryns shields like Haven will also trigger this effectively turning Hildryn into a walking shock mote. This used to be really fun with the old Unairu's damage reflect, just being able to walk into a group of enemies and killing them with all the reflected damage.

Harrow is probably one of my most played warframes. He has great survivability and some very strong weapon buffs which make weapons that have usability issues a lot more viable and fun to use. It is a lot easier to play Harrow if you only focus on modding for solo gameplay rather than team support. Missions I would typically use Harrow for would be Disruption and Survival, and mostly focused on duration and strength.

Condemn works great for quickly freezing important targets like Demolysts in place for easy headshots. It still won't hold them for long but with Harrow's fire rate buff and crit chance, you should be able to kill most Demolysts very quickly. I would not bother using the condemn augment, especially with how easy it is to get energy now.

Penance has a useful interaction with shield regeneration mods like Brief Respite and the Augur set, If you don't have active Overshields with Brief Respite equipped, the shield regen will prevent your shield gating from tripping making it a lot safer to use around enemies. The Augur set is a little better if you can fit one of the mods as it can prevent penance from triggering shield gating even when cast with active overshields. Penance also gives a large boost to your weapons fire rate and reload speed making certain weapons much more usable without wasting mod slots.

Thurible is mostly just used to keep your own energy up and allows you to run around with negative efficiency and always have a cast of condemn ready for emergencies. I don't find Warding Thurible to be very useful.

Covenant and the Lasting Covenant augment are what I usually try to mod for on Harrow. This is mostly for precision weapons or if you loadout is focused around headshots with arcanes like Arcane Pistoleer or single target weapons. If your team is nuking or CC focused with Gloom/Shock motes, it's best to find a group of enemies alone to get your full buff or find a couple exploding barrels to take self damage.

For weapons, generally I like to use snipers, Penance and Lasting Covenant help to work around the lower fire rate and reload speeds, and the Sporothrix is pretty fun with this setup and its Volatile Variant augment. For pistoles, Arcane Pistoleer and anything that's full auto and has a high fire rate like the Twin Grakatas. As long as you're getting headshot kills, it's like running around with an infinite ammo fissure buff. The melee weapon typically doesn't matter, Rakta Dark Dagger to quickly refill shields or the Furax with Amalgam Furax Body Count if you want to have even more fire rate on your pistols. It's also probably one of the few warframes where spearguns like the Scourge and Afentis can be useful.

Probably one of the harder warframes to farm because a blueprint is on a Rotation C mission. If you see her as a reward for the Duviri Circuit it can save you a lot of bad RNG trying to get her Neuroptics from a Survival mission.

Her abilities make most difficult missions doable even while solo, and my favourite use for her is to use Mallet to quickly spawn a Sister of Parvos. Her kit is pretty versatile giving mild CC, damage, invisibility, and even energy regeneration which can also be granted from Octavia Specters.

Bromius concept art from the Soulframe Discord.

Wukong kind of got hit kind of hard by that ammo nerf. The clone tends to burn through ammo fairly aggressively, and his abilities aren't really focused towards killing enemies.

He's still a good choice for missions like Spy and Capture, and his abilities offer a lot more survivability compared to other frames.

If you want utility from his clone, give him a weapon with Radiation/Cold for some minor CC, or something that's good at applying multiple statuses for CO mods.

Cloud Walker is great for avoiding enemies and covering large gaps like the ones on the Gas City tileset.

Defy greatly increases Wukong's armor, it's kind of disappointing that this ability is still nerfed from the Helminth, and would have been a great alternative to requiring shield gating for endgame content. Obviously armor only goes so far, but for non endurance missions the survivability from this ability would have been more than enough.

Iron Staff is pretty disappointing, it seems it's only use would be to get one of the buffs that require you to have a channelled ability active.

If you're going to touch content on the Zarmain, you might need to invest a bit in an amp if you haven't already so that you can deal with Thrax/Void Angels. Madurai/Unairu are the two focus schools I'd recommend for these missions if you're after Void Angles.

If you struggle at all with steel path, play the daily 5 missions in public. It's an easy way to get the steel essence required to buy arcane adapters for your weapons, and a chance to get the Deadhead/Merciless/Dexterity Arcanes if an acolyte spawns.

If you don't have Galvanized mods yet, you can pick them up from the Arbitration Store for Vitus Essence.

If they're on PC, they can try clicking the settings cog on the launcher and verify their downloads cache. (No clue how to do this on console)

The only other thing I can think of checking is to make sure your Network Ports are set to different values if you're on the same network. Warframe will usually pester you if multiple people on the same network are using the same ports, so this probably isn't your issue, but you can find it under Settings > System > Network Ports. Any value is fine there as long as it is set to something different from the settings used for your game.

If you still run into issues, you might have to make a bug report on the forums to let them know that certain dynamic/interactable objects, like the hatches Zaramin, are indestructible while your partner is playing as a client. Still no clue what's going on with Lua but at least that one seems pretty minor because it's in a vault where only used for that mission and it's in the state for an alternate path rather than a state preventing completing the mission.

They somewhat recently "reworked" Cephalon Fragments/Somachords/frame fighter to make them less visible if you've already completed them. If you've already scanned them, they won't show up on your map* and will appear translucent.

On higher level missions (planets that are (+25) I find Somachords/Cephalon Fragments start to show up on the map again even if they've already been completed for that planet.

Celphalon Fragment
Somachord

Just for completeness, here's an example of the scanned versions on a Jupiter mission which do have working mini-map icons:
Cephalon Fragment
Somachord

You could always remake the meme with Note Beacons:
https://imgur.com/YLrog93
You might need a 3rd beacon to fit whatever word you put there though.

Other than that, either an Archwing Articula or or one of the model ships that Baro brings around seems to go with the theme of putting something on a pedestal.

"Auto-Melee will persist if the first melee attack in a mission is a Warframe ability (ex: Kullervo's 1) "