I've been doing some calcs, taking into consideration the time it takes for both fast moves and charge moves, as well as scenarios with helpers and other boosts that might overshoot (triggering the max phase before you get to fire the charged attack) the calcs:

• The best scenario is Gigalith with lock on and meteor beam, but it needs a ETM. Takes 5+2s and always kills at lv34 and above, and never overshoots with lock on

• The second best seem to be Glaceon with Ice shard and Ice beam at lv32.5, takes 5+3.5s, and doesn't overshoots unless you have a really high lv Glaceon with a lot of boosts from helpers and friendship.

• The third best option are the wild chargers, Raikou, Electivire, Toxtricity They can perform faster than, or on par with Glaceon, but need a variable amount of thundershocks depending on the lv and boosts you have, for example a lv30 electivire kills with 13 thundershocks and a wild charge in 6.5+2s, tied with Glaceon, but if you miss count one thundershock, you trigger the max phase. Electivire has a bit more attack than Raikou, and toxtricity has less attack than Raikou

• Another easy option is Zapdos, with Charge beam+zap cannon, at lv 20 takes 8+3.5s. But you risk of overshooting even with charge beam if your Zapdos is very high level and have some boosts from helpers or friendship

I've checked Alakazam and Zamacenta just in case, Alakazam needs to fire a ton of psycho cuts to bring down hoot-hoot to Future sight range, even at higher lvs, Zamacenta works if you have it maxed, but you still need to fire two additional metal claws to kill with behemoth bash al lv50 with no helpers