Details of reaching AR T20/T21 as f2p

The key being that once you clinch T21, it's hard to kick you out and the only competing is ranks until IS adds T22-25, and I think they will.

Reaching T20: I've calculated this out. For lift gain of +100 to +160 per battle, breaking all aether structures will enable 14 raids. The gap to T21 is +1200 lift. So without scoring bonuses from a mythic, your defense would only be allowed to lose 200 lift all week if you did perfectly on offense.

During light season: Let's look at three thresholds.
5 losses for -80 each: offense needs to gain 1600 lift. +120 per battle x 14 battles = 1680. So it's doable with unmerged Eir with two light-blessed, and offense could still afford to lose 4 units over the course of a season.
6 losses for -80 each: offense needs to gain 1680 lift. So for unmerged Eir with two light-blessed, we're looking for
7 losses for -80 each: offense needs to gain 1760 lift. +130 per battle x 14 battles = 1820, so it's doable with unmerged Eir + three light-blessed or half-merged Eir with two light-blessed with 60 lift of wiggle room.

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Reaching T21: for lift gain of +120 to +160 per raid, one season allows 13 raids. The gap to T21 is +1600 lift. That's impossible without mythic hero scoring bonuses.

So let's look again at some thresholds for defense scoring and the minimum lift gain per battle if offense doesn't lose a single unit.
5 losses for -60 each: +147
6 losses for -60 each: +151
5 losses for -80 each: +154
6 losses for -80 each: +160
7 losses for -80 each: +166

And let's look at scoring:
Unmerged Eir with four light-blessed (including bonus unit, if she's not the bonus unit): +140 (insufficient)
Eir+10 with four light-blessed: +150
Double unmerged Eir with three light-blessed: +160
Eir+10 and Eir+0 with three light-blessed: +170

Which means most f2p who want to get into T21 will probably need to find ways to win with double Eir on the team, which doesn't sound so impossible when there exists a certain defacement by the name of Surtr. Also, unless we're getting a third light blessing for free it is necessary to pull one, which wasn't so hard while hero fest was on, plus colourless is probably going to be quite good for the next 2-3 legendary banners.

One last note: the next few weeks will be easier to get into T21 than in subsequent weeks because T20 will be less populated, which means needing to make fewer defenses.