A full-grown male is
about 1.8–2.1 metres (6–7 feet) long, excluding the 1-metre tail; he stands about 1.2 metres high at the shoulder and weighs 170–230 kg (370–500 pounds). The female, or lioness, is smaller, with a body length of 1.5 metres, a shoulder height of 0.9–1.1 metres, and a weight of 120–180 kg.
From :
https://www.britannica.com/animal/lion
If I'm not mistaken, if all the lions were arranged into a square somehow, we could determine how many rows & columns it would have, by getting the square root of the army size, & thus, since each row/column is a lion, get a rough idea of how much space the lion army takes up.
With that info, you could learn the range value you need to hit the whole lion army at once.
('course, having torso thickness might also be valuable for determining size of a row/column in another measurement, but we can work with this.)
So, since the square root of 1 billion is 31,622.7766, there's approximately 31,623 rows & columns of lions.
If they were all as short as possible, all lionesses & we ignore the tail, perhaps because the lions always keep it down or such, it's 1.5 meters.
Times 31,623 & we get 47,434.5 meters or only 47.4345 kilometers.
If they were all as long as possible, with a fully outstretched tail, that's 3.1 meters each.
Times 31,623 & we get 98,031.3 meters or only 98.0313 kilometers.
If I'm not mistaken, the average individual lion length in this case is 2.3 meters, so with that in mind....
31,623 * 2.3 = 72732.9 meters or 72.7329 kilometers.
Even with the lions as long as possible, a single row or column of them is still less less than 100 kilometers. Tens of KM range can reach that far!
But what about the whole group?
Well, let's look at the High End for Area. To get Area, we'll multiply the measure of one side (In this case, a row or column.) by another side, & since it's the High End, we'll use the highest result of
98.0313 km
9610.1 km^2 (square kilometers) As one of Wolfram's comparison's so helpfully says, this is about 46% the total area of the country of
Wales's ( ≈ 8023 mi^2 ) area.
....'course, perhaps it's better not for the lion army to be placed in Wales, since besides the lion army taking up half that part of the UK's area, Wales's last estimated population (from 2019) is only about 3.136 million, which is probably too few people in too little area for 1 billion lions to be present.
Anyway, that should show that even with the lions at their longest, & even giving space for the whole of one another's tails, any character that can attack an area of 9,610.1 square kilometers at once should USUALLY be able to attack the whole lion army at once, assuming the lion army is not too spread out.
My favorite Pokemon is
Mew.
Range: Standard melee range. Planetary with projectiles and abilities.
Among the potential AoE moves it has listed on our profile for it are Dazzling Gleam, Dark Pulse, Surf, Blizzard, Rock Slide, Overheat, Bulldoze, & possibly other moves.
DIE, LIONS.