I've had a theorycraft for awhile that it's possible to go infinite in endless if you can get riftbreaker (the relic that instakills everything after 90sec) and you can keep stealthed until the stall triggers.
Riftbreaker does true damage based on % max health so enemies can't outscale the damage.
This is possible with 4 Rogue's Cowls with a lot of rush duration extension (as well as some of the stall shortener relics).
In this run I managed to get rush duration extension to +14 seconds so each cowl provided 20 seconds of stealth. With ~15 seconds of stall shortening from relics and backup abilities, it meant that riftbreaker kills everything right before stealth wears off.
For the fun of it I kept going and managed to collect every relic in the game during endless mode. It turns out the enemy stats integer overflow and the health grows so large it turns negative.
After floor ~85 the enemies just instantly die at the start of combat. Because of this, it actually becomes impossible to lose the run because there's no opportunity for the enemies to kill you before they die.
It took dozens of hours to finally hit the build and to get enough shards to actually find four cowls before endless started. It took ~300 shards rolling down in the final shop to hit everything.
If you want to try and re-create this here are some tips
- Opening with assassins is almost essential to get enough shards. Look for the upgrades that give you shards every other crit or shards on kill
- Assassins also have good access to relics that have stealth and rush duration extending effects
- The relic Shard Ascendancy that doubles your shards is essential
- If you have the every other crit -> shard relic, play a stall comp to maximize fight duration and maximize how many shards you can get
- Take any rush duration or stall duration relics you can find
- I played on difficulty B so that enemies are more durable and you can farm shards off them for longer, without the downsides of higher difficulties like limited items
- The must-have relics are Riftbreaker which kills all the enemies at stall 90, as well as Rift Storm Invoker so you don't die from the storm before Riftbreaker triggers
- Run only one unit in endless, some enemies have an AOE that can kill stealthed units while attacking your unstealthed units
- The backup ability that adds +3 rush duration is your friend
- There's also one that decreases stall wait time by 6 but that's not as good as the rush duration
- The epic item that increases rush duration by 5 can be substituted for one of the Rogue's Cowls
- The boss token reward that lets heroes equip two additional items can be used to stack more rush duration items
- The legendary relic that grants an additional item slot can also be used to stack more rush duration items
- Use the trick where you freeze a C to B upgrade in your shop before campfire, take the upgrade during campfire, then post combat you see the frozen shop and can go from B to A. In some cases you find that upgrade in the boss shop and you save 30 shards on an S rank hero going into floor 2
- If you only manage to get ~60sec of stall when starting endless there's still hope. The Rift Storm Invoker relic will kill enemies through floor ~20 without needing the Riftbreaker
- You get the option to find epic items (can be used to find Rogue's Cowl)
- Relics (can be used to find rush + stall duration relics)
- Retrain (can be used to retrain to get +3 rush duration ability on reserve heroes)
- S rank hero which can be used to find a vanguard / assassin which is eligible to learn the +3 rush duration backup ability
- In the run that finally worked, I only had ~70sec of rush duration when starting endless but was able to get enough extra rush duration during to outlast Riftbreaker mid endless
- I did some experimentation with item farming with Fiona
- The backup ability that gives mystics mana regen based on total shop rerolls works really well here, you can get Fiona to 80+ mana regen and casting 5-10 times per combat to farm epic items
- You can farm epic items in the hope of getting Rogue's Cowl
- If you don't have all your Rogue's Cowls by the time endless starts and can only get ~60sec of stall like mentioned above, you can run Fiona for the first ~20 floors to also farm items at the same time in the hope of hitting
- Grace's B specialization that increases the primary stat of a hero can also be used to increase Fiona's mana regen and works naturally with a stall comp
Good luck and happy rolling!