I would have to disagree with maxing Half/Deadline Slayers. At first glance, the large combined increase of +250 Mel-Atk (maxed Half/Deadline Slayer) may seem appealing, but there are actually more factors that we must consider. First of all, Half/Deadline (as well as PP Slayer and Crazy Beat) gives you a FLAT Mel-Atk bonus. This means that they cannot be further boosted by buffs such as Shifta Drink (x1.20), Team Tree Atk Up (up to x1.20) and Shifta (up to x1.197 without Te, and x1.24625 with Te).
On the other hand, Mel-Atk gained from Melee Up 2/3 are added directly to your BASE Mel-Atk. This means that they affected and can be further boosted by the buffs mentioned above. (Note: this also applies to any stats gained from equipped mags as well). Now, a quick math to visually show what I mean.
Let's make our base Mel-Atk is 700 and that our weapon gives us +800 Mel-Atk.
With Melee Up 2 (at max), we will get an additional +75 to our base Mel-Atk for a total of 775. If we get a max Team Tree Atk buff (x1.20), Shifta drink (x1.20) and normal Shifta (x1.197), our total Mel-Atk becomes 775 x 1.20 x 1.20 x 1.197 = 1336; if we include our weapon, it'd be 1336 + 800 = 2136. Once we use OL, we will gain an additional Mel-Atk boost of +200 (Crazy Beat), +26 (Halfine) and +40 (Deadline) for a total Mel-Atk of 2136 + 200 + 26 + 40 = 2402 Mel-Atk.
Now, let's see how it is with maxed Half/Deadline Slayers. Our Mel-Atk with all the same buffs above would be 700 x 1.20 x 1.20 x 1.197 = 1207; if we include our weapon, it'd be 1207 + 800 = 2007. Once we use OL, we will gain an additional +200 (Crazy Beat), +100 (Halfline) + 150 (Deadline) for a total Mel-Atk of 2007 + 200 + 100 + 150 = 2457 Mel-Atk.
As you can see, with a max Melee Up 2 build during OL, we're only behind a max Half/Deadline Slayer build by +55 Mel-Atk only. On the other hand, while we're off OL during for the 30s downtime (or even longer if you died while using OL), we see that we're actually +136 Mel-Atk HIGHER than the maxed Half/Deadline Slayer build. And this is including only one max Melee Up; if we had maxed Melee Up 3 as well, the small gap during OL will become even narrower while the gap off-OL will become even wider. Additionally, unlike Half/Deadline Slayers, the Mel-Atk gained from Melee Up 2/3 are unconditional. Unconditional buffs are what you'd want the most!!!
Keep in mind that these calculations were done without Te Shifta (x1.24625) and the Shifta Drink buff that NA will eventually get (x1.50 instead of x1.20). The point is, going Melee Up 2/3 is by far the better investment as opposed to maxing out Slayers. Now that I've given you a bunch of maths and explanations, like Gerbilios said, the choice is yours at the end on what you want to do. I'm all for player choice and playstyle, but I do want to ensure that you are well-informed before you make such decisions.