This guide will be current as of March 2021. Shadow has a good guide. I will be working off you have picked your focus school. Now the 250k focus limit is partially correct. That is the starting point. You also get an additional 5k per mastery rank you are. This is a soft cap. You can go beyond this. There are 3 methods of gaining lots of focus. I will state early that it does require some advanced items to get the most out of this.
With the helminth system. You can always subsume regular Equinox. Putting her rest ability on whatever frame you want. You can also use Equinox. Using a 265% range Equinox(prime) with her rest and rage augment. With about 155% duration. You also want lots of enemy radar. The weapon you want is the skiajati. Upon doing a finisher attack you will gain 5 seconds of invisibility. You will lose that if you do a non-finisher attack or activate an ability Now for the other items that will make this easier. The affinity booster smeeta kavat with the charm and tek enhance mod, mod preparation, rank 5 arcane energize, and arcane trickery. Energize gives 60% to get 200 energy every time you pick up an energy orb. Trickery gives 15% for 30 seconds of invisibility. The map you want is adaro on steel path difficulty. You unlock the steel path by unlocking all the nodes. The strategy is fairly simple. In her night form Cast her rest ability and get stealth finishers. Try to maintain a 500% with it. You especially want to keep it going when you collect the convergence orb.
Capturing eidolons grants brilliant and radiant eidolon shards. Which are worth 25k and 40k respectively. With both good gear and squad you can easily gain all the focus you will need. The focus you gain via this method doesn't count against your focus limit. You can gain as much focus in a day as you have shards. To capture an eidolon requires you to remove its shield with your amp. You will know it is still shielded if you see purple over its health bar. After getting rid of all its shield it will flash red. I highly recommend you do a 777 amp. Though it is up to you. Then use a different weapon to destroy the synovia. The 1st one has 4 the other 2 have 6. You repeat the process of taking down the shields and destroying the synovia until all of them are destroyed. It will get up one final time in which you can damage it anywhere. To capture the 1st eidolon requires 2 charged lures. The other 2 eidolons require 3 charged lures. To destroy the synovia and damage the eidolon in its final form. I say the best option right now is the necramech voidrig with its 4th ability guard mode. Which is an exalted weapon. When properly modded will make short work of them.
You can use whatever frames and weapons you want. Though you will want ones that don't rely on their 4th ability. The mode you want is elite sanctuary onslaught. At the start of each round, a convergence orb is automatically applied to you. That can scale up to 16x. The more rounds you go.