Hello, I have soloed the Grendel Missions multiple times in the past, and I can tell you the easiest and most accessible way to do them is just one word. Garuda

This applies to all the missions, just use Garuda and the rest of your loadout really doesnt matter. Just make sure your Garuda is Rank 30 with no Mods Equipped so you start off with Max Energy

To explain why its this, it comes down to pure Synergy. Garuda is a deadly Warframe on its own, her 4 Will be able to mass stagger if not outright kill most of the Enemies, her 2 will allow you to Keep Alive in any situation as her Healing Scales with how much health you’re missing while also allowing you to use your 3 to Refill your energy on Demand, her 1 is a Neat Survavibility Tool but since youre using your 4 which gives Invencibility it instead becomes a Nuker, use her 1 and Let Enemies shoot at the Shield, when a Tanky Unit like an Eximus appears, using her 4 in Combination with throwing her 1 is a Death Sentence regardless of Enemy Level

As of the explabation, there is a strategy to be maintained here, yoy must keep her 2 Active and Constantly use your 3 and 4. BUT if you have access to the Helminth System and Sevagoth subsumed, give her Bloom instead of her 2 and you can Play the Mission without a care in the world, her Ability’s Damage will heal you

Best of luck Tenno!