Solo Grendel farm

2 Star Gaming did a video on this a long time ago, pre-Eximus rework. Not sure how well it works with the new Eximus units, they may have done a follow-up video, I'm not sure.

The Excavation mission can be cheesed with Limbo, but it's really really boring. As far as I'm aware, you don't have to actually complete any excavators, just get a certain amount of Cryotic. So just run around as Limbo in the Rift, activate the Excavators, and let them get blown up for a small amount of Cryotic. Rinse and repeat, watching out for Eximus units that can hit you through the Rift. Maybe subsume Banshee's Silence if that's a problem for you: while it's active, Eximus can't cast their abilities that hit through the Rift, and Silence itself works across the Rift.

For the other two, there's a little known trick. The game says that all mods are disabled, but that's not entirely true. Warframe Augment mods still work for some reason. So the strategy is: grab Hildryn, subsume Chroma's Elemental Ward over her 1 or 4, and make sure your Emissive colour gives you Electric Ward (Purple colours usually work for this). Then, equip Hildryn's Blazing Pillage mod. Activate Elemental Ward, activate Haven, and start spamming Pillage. The combination of Pillage's armour/shield strip, Haven and Blazing Pillage's heat damage, and Elemental Ward's interaction with Hildryn is enough to shred the mobs at the level of Grendel's missions. Just keep an eye out for Eximus units, as their Overguard is the biggest unknown since it didn't exist when I did this strat.

Finally, don't be afraid to ask for help in Recruit chat! Only the mission's host needs to have a Grendel locator to access the mission, iirc, and plenty of players will jump at the chance to farm Grendel in a group, or at least get a second copy to subsume to the Helminth.