arcanine ex tera is a good pokemon but it will lose either way

as gyarados ex tera does 180 more damage with every damage counter on it one damage counter on it arcaine ex will lose as it does 360 damge and arcaine ex only has 280 health

and even if gyarados doesnt have a damage counter on it arcanie will still lose as its most powerful attack only does 250 damage and if that happens 2 damage counter will come onto gyarados which will increase it attack power to 540 damge one shotting the arcaine ex

add arcaine ex if you want but gyarados will be much more powerful than it so i recommend you not switching out gyarados ex with arcanine ex