Za warudo!

Time stop, Forcecage, and Cloud Kill. Deadly combo mentioned by the Dungeon Dudes on YouTube.

Good thought, but it doesn’t work. Forcecage blocks spell effects from being cast into or outside of it, which means cloudkill has to be cast first. Unfortunately, doing so affects another creature and ends Time Stop; but if it hasn’t left Cloudkill by the time your turn comes around, you can certainly do that Forcecage!

Honestly, I just don’t think this spell is worth a 9th level slot. Not worth it to just buff yourself.

You could cast cloudkill just outside forcecage, then let gravity or wind move it into the cage when time resumes.  Although I suppose that depends on whether you count the cloud itself as a spell, or the effect of a spell. 

Alternatively: Timestop > use  telekinesis to move 1000 lbs boulder 10 ft above target's head. ...then, for good measure, an 8th level fireball just beneath their feet as you release the boulder.

In any event, time stop may have sub-optimal use in combat for a ninth-level spell (see meteor swarm for when you absolutely, positively, need to Wreck-It!™ right the heck now), but it's still incredibly cool.  Depending on how lenient your DM is, I see this spell having all sorts of hi-jinks opportunities outside combat.  My sorcerer would use it just to place hats just above people's heads

The prison can be a cage or a solid box, you choose. And only the box has the condition about preventing any matter or spells from passing through.

So, yes, you can definitely use the cage with Cloudkill. 

A prison in the shape of a cage can be up to 20 feet on a side and is made from 1/2-inch diameter bars spaced 1/2 inch apart.

A prison in the shape of a box can be up to 10 feet on a side, creating a solid barrier that prevents any matter from passing through it and blocking any spells cast into or out from the area.

Cloudkill starts its effect on the creature's turn, not the caster's.  It shouldn't end Time Stop prematurely.

What really annoys me is that Delayed Fireball is a concentration spell, so no more casting as many as possible to nuke that one particular area.

basically

Cloudkill takes effect as soon as a creature enters it and starts their turn in the cloud. But it does move 10 feet a round.

You can't even use it to buff yourself effectively because of concentration rules. I rule that any damaging spells my players cast do not take effect until time stop ends, and allow them to concentrate on any spells they cast while under the effect simultaneously. But RAW it's a pretty underwhelming 9th level spell, especially compared to its power and utility in previous editions.  

Ah yes i see a fellow intellectual

this, ice, knives and vampiric touch. then you have dio

If I could thumbs up this I would

The rules for this spell are so weak. You mean to tell me that because I pick pocketed a guy now a 9th level spell just ends? what magical force is causing this to happen. that's just to much. 

although you could pretty much get into any fortified location with this. do a couple of passwalls and disintegrate the vault.

So could you use this in conjunction with Delayed blast Fireball cast every turn while under timestop to make one truly big explosion?

One read-as-written way of pulling a DIO is to get a large rack of sharp objects, set it up in front of your target and then Thunderwave it into them. Et voila, one monster kebab.

Nice, Nice, Nice, very nice caesar-chan

ZA WARULDO! TOKI WO TOMERE!

A fun way to use Timestop would be casting Greater Invisibility during it, and casting Fireball. They would have no way of knowing where the fireball came from, and it would just be hilarious watching them try to find out

I can't wait to stop time, pick somebody up, and move them two steps down the stairs they are climbing right before I go back up the stairs and pose.

Quote from prof_narwhal64 >>

I can't wait to stop time, pick somebody up, and move them two steps down the stairs they are climbing right before I go back up the stairs and pose.

 Would not work:
"This spell ends if one of the actions you use during this period, or any effects that you create during this period, affects a creature other than you or an object being worn or carried by someone other than you."

Picking someone up and moving them definitely counts as an action that affects a creature other than you.