As my membership to the Oxford achaeology department has expired and since the Celts didn't really have a true written language and that the vast percentage of written accounts were made by their enemies, it's been difficult finding reliable information in books or the internet. So here it goes 1) The Chemical Cocktail - Mandrake is just one of the myriad of plants/natural substances that will render someone immune to pain. Since Celtic Druids spent lifetimes studying and worshipping nature, I don't think it a stretch to deduce that they could have been able to brew some sort of substance that would improve reaction time and protect the user from pain (aspirin is just one such substance that provides pain resistance and is naturally occuring). Modern medicines may or may not be more effective than ancient ones in all aspects. It's a tough argument to make since we really don't know what the ancients had and that people still die from things like TB and rheumatic fever.
2) The Instant Kill - In 6 years in the military, I never had the displeasure of engaging in CQC, been shot at and shot back, but never hand to hand. I have though, studied warfare for over ten years and judo/aikido for over twenty. In that time, I learned many things. The most important of these is that the instant kill is the best way to dispatch an enemy yet the most difficult to achieve (funny how that works). Even today, shock/blood loss is the combat medics worst enemy and most advances in trauma medicine have been geared toward that. Alot of the advantage of combat is related to what I like to call the OMG factor. It's the reason why green troops run and veterans stand and it also severely affects the way a soldiers will deal with a shocking situation. Picture the following: You are a Hastati in the Roman army. You've had some training and have put in time as a Velite so you have some combat exp. though you are still 'green' as the true grimness of hand to hand has not yet touched you. You're standing in line and watching a bunch of naked crazies jumping up and down on a hillock screaming bloody murder. They begin their charge (sword and shield in hand). Your skirmishers are first. They run in between your ranks, javelins in hand. You watch as your enemy is pelted by javelins. Some fall with grievous wounds (head, neck) others take javelins in legs, arms, shoulders etc. Some of these fall and just get back up, the others shrug off their wounds as if it were nothing and keep coming. In a rather brash action, one of these naked psycho's pulls a javelin out of his leg and hurls it toward you. You raise your scutum just in time to block the missile which rocks you back on your feet as it impales itself into your shield. The Velites are now falling back through your lines. The centurion orders your unit to ready pilum. At about 75 yards you throw your first. Once again, some fall, others receive bad injuries but don't seem to notice. You throw the second at 50 yards, same result. The enemy is close now and the usual stoicism of your tent mates is replaced by the muttered Roman equivalent of OMG. The centurion orders preparation to receive the charge. You ready your shield and sword and begin to have serious doubts about your ability to survive this battle unscathed. You pick out your target (who looks to be a good 6 inches taller than you) as he closes and brings his sword slashing down as you bring your shield up. The block leaves his body open and you stab him authoritatively in the gut. Instead of recoiling in pain and falling down like he's supposed to, he just looks mad and while you're doing this he's converting you from a righty into a lefty. Believe it or not, being mean and nasty in combat accounts for alot.