There really isn't, the alpha pokemon you encounter will always be a certain number of levels higher than your highest level pokemon. On the bright side though if you actually manage to catch one (which you can do believe it or not it's just not easy by any definition of the word) it will be a massively valuable asset in battle.
Theoretically, for any Pokemon game, unless a Pokemon is specifically labelled in-code as uncatchable (or in the case of Pokemon Essentials, if its Catch Rating is 0, which means only a Master Ball can catch it), there's always a chance to catch one. It just so happens that for something like this scenario, the chance is pretty slim without throwing the kitchen sink at it.
For that scenario xeno had, Arcanine has a catch rating of 75. Let's assume he's at full health, since he's a much higher level than the team. I'll be generous and put a BURN on it, since status conditions can help with catching, and the MC always has a Cyndaquil (or equivalent evo). I'm not
entirely sure what Pokemon Essentials (or this specific fangame, for that matter) uses for the catching formula, but since I can clearly see the Gen 5 spritestyle, I'll assume that. We'll assume the grass is thick (yes, this is important). And, for the last assumption, let's assume that xeno is using just regular standard PokeBalls.
I'll save the long and drawn out math and just draw it down to the results:
There is about a
9.627% chance to catch this Arcanine in one throw.
This chance goes to about 50% if you use 7 balls in a row.
At 30 balls in a row, you will have about a 95% chance.
If you use Great Balls, the chance per throw grows to about 13%. For Ultra Balls, it's about 16%.
