The trick is simply when you look at it from face value. Alot of people would say gai, neji or lee by themselves could give deidora a fight and they are right, therefore all three is overkill?, no. The fact that all three are basically taijutsu users alone shouldve rang a bell in your heads.
Ive noticed that when people give answers to vs threads, they usually just take into consideration numbers, biased towards one character and only take into consideration their abilities and totally ignore the opponents, noticed that alot of vs threads are biased towards leaf ninja's especially when their opponents are akatsuki.
Anyway back to this. The fact of the matter is that gai, neji can lee will just be bomb raided, ive taken into consideration their abilities and from that, they cannot go on the offence at all. They would at a massive disadvantage from the start. Im so confident that c2 alone should be enough to win this fight, but if he wants to finish it off quickly, c4 would undoubtedly end it. Sure neji would mostly likely see the bombs but they are all bound to inhale it, even sasuke did, but what saved sasuke was the fact that he came up with a briilliant strategy but more importantly he had raiton to neutralise the bombs and neither of these three opponents do, therefore neji seeing it isnt all that helpful.
Also the perfect opponent for deidora at that time was sasuke due to his sharigan, strateic thinking, lighting release and more importantly cm2, because its with this that made him able to actually attack deidora.
What i was trying to prove in this thread is that although deidora arguable doesnt even make the top20 strongest characters, he can kill almost anybody