We are a large party, all level 6:
Storm Sorcerer, Shadow Sorcerer, Lore Bard, Dual wielding DEX fighter, Thief, Moon Druid, Forge Cleric
We are repeatedly running into a problem encounter, that we've had to run from or take NPC losses, only killing less important enemies.
It's a group of hobgoblins all mounted on wargs or greatwargs.
Their leader has heavy crossbow with 3 shots (and melee weapons?).
Their mage will turn invisible and get off warg, throws level 4 spells at us.
And there's two more mounted with lances and greatswords as a buffer.
Now as a Forge Cleric I have massive AC. If the mage wasn't around I think we could take them.
DM seems to do lots of things that bypass my AC, fireball, ice storm etc..
I want to close the distance but they stay back and I don't have great range. Everything seems to pass my spell saves too.
Fairie Fire is a good idea but Bard doesn't have it, and when Druid missed a session DM said she "vanished" so I don't know when she'll be back.
I do believe I understand 5E stealth rules. My DM didn't understand and may still disagree. He is coming from other editions and is Homebrew happy. It's a point of friction.
What are good strategies?
How do we get this invisible mage so we can make with the heroics, and story progression?
Best Answer
Prevent it from going unseen in the first place
The most straightforward way of hurting something that is invisible is by preventing it from going invisible in the first place. If the mage is within 60 feet of you when it casts Greater Invisibility, both your Bard and the two Sorcerers can attempt to counterspell him. This is by far the best solution, because it makes the mage waste a fourth level spellslot without actually going invisible.
If your sorcerers have Distant Spell and don't mind silly gimmicky layering, you could in theory Counterspell them from further away with Distant Spell. While Counterspell's trigger won't double in range, it also doesn't specifically say that you have to counterspell the creature that triggered your casting. So you can have one person prepare "I cast this cantrip when the mage starts casting", and then that becomes the second person's trigger for counterspell, at which point you can metamagic it to 120 feet and counterspell the enemy mage from a larger distance, but this seems extremely silly and rulebendery. Alternatively, your DM might just rule that Distant Magic simply works by default with Counterspell, changing it to 120 feet.
If, however, the mage is too far away, then you have several characters who are good in AoE spell damage. Every time the mage takes damage, they'll have to make a concentration check to remain invisible. Simply throw all your AoE spells in the spot where you last saw the mage, capture as many of the other Hobgoblins in the blast as well, and go from there. As soon as the mage fails their save, they'll be revealed again.
Faerie Fire would not really help you because it's vastly outranged by crossbows and fireballs (it has the same range as counterspell), so there's little reason for the enemy mage to come in range of that spell if they weren't already in range of the counterspell in the first place. It might be useful if you have Distant Spell and are then able to get him in the cube from 140 feet away, but it's vastly inferior to preventing them from going invisible in the first place.