The only physical attackers Chansey would be going up against have already had their attack lowered or will be coming out after Chansey has defeated a special attacker, in which case Chansey should be at 100% HP with a Substitute and +6 evasion. Whether against a special attacker or a crippled physical attacker (and it doesn't take much effort to neuter a physical attacker when even something like Mega Aerodactyl can't break a Chansey Sub at just -1), the only way Chansey could possibly be in danger is through repeated critical hits or secondary effects, so being faster and able to set up a Substitute and evasion before opponents can move does in fact make it better defensively. In the scenario where Chansey's out against a physical attacker that comes out second or third, its Substitute is at the very least an extra 81 HP an opponent must take out to KO it. Full investment in HP only gives Chansey 32 more HP and doing so leaves it too slow to put up a Substitute first against a bunch of annoying OHKO and status users.
Just play the Battle Tree more. If you have a team that can easily win 50+ in a row, that's basically a Rare Candy's worth of BP every 15-20 minutes (plus the additional Rare Candy/Bottle Cap/whatever else you get from the attendant when the streak ends). If you're trying to get something to level 100 so you can use bottle caps, that's not going to be much slower than grinding through the Elite 4, and it comes with the added bonus of making you better at actually playing the game.
Appreciated for the feedback.
I'm a bit confused by what you said, are you saying it is better to grind through battle tree with the team to get rare candy and BP and bottle caps every 20mins rather than going through elite 4?
So I want to level all my winning team pokemons to 100 (lv 84, 53, 54 respectively), I need about 109 rare candies.
And you are saying by playing battle tree to 50+ to get the candies enough would be actually much more worth it?
I would need to play about 34 hours at the minimum straight assuming I can win and play like a machine consistently to get that result.
Man, that is a lot of work still, even if it is much much shorter than doing it traditionally via elite 4 or FC route that is popular on the internet.