If all else fail...

-- ...bruteforce the wiener out of it!

-- So, I've recorded the total EXP values needed to advance a character to the next level in Disgaea 1.
-- From a coding standpoint, it is more effective to implement a formula for total EXP, and then calculate ENEXT as a difference.
-- Otherwise, you'd have to calculate a few thousand ENEXTs for every mob in a particularly deep dungeon,
-- which is not a smart thing to do when your game runs on a PS2
-- The EXP curve behaves very differently before LV99 and after LV99. Let's start with the first part.
-- Assuming that EXP is given by a polynome, I've taken a few values as control points and obtained this approximation:
--    EXP = 0.0100003 * LV^4 + 0.289923 * LV^3 + 1.05555 * LV^2 + 1.14207 * LV + 0.50246
-- Most likely, the actual formula is an integer expression of a similar nature. I assumed it was something of this form:
--    EXP = floor(((((A * LV + B) * LV + C) * LV + D) * LV + E) / 1000)
-- This program tries different values of A..E in the neighbourhood of the ones above and compares the results with actual EXP from the game.
-- As it stands, the EXP function for 1<=LV<=98 may be perfectly represented by this expression:
--    EXP = floor(((((10 * LV + 290) * LV + 1050) * LV + 1271) * LV + 486) / 1000)
local exp = {
    3,
    9,
    22,
    43,
    75,
    121,
    184,
    267,
    373,
    508,
    673,
    875,
    1117,
    1404,
    1740,
    2132,
    2585,
    3104,
    3696,
    4365,
    5120,
    5967,
    6912,
    7962,
    9126,
    10410,
    11822,
    13371,
    15066,
    16913,
    18923,
    21104,
    23466,
    26019,
    28771,
    31733,
    34915,
    38329,
    41984,
    45891,
    50062,
    54508,
    59241,
    64273,
    69616,
    75282,
    81285,
    87636,
    94350,
    101439,
    108917,
    116798,
    125096,
    133826,
    143001,
    152638,
    162750,
    173353,
    184464,
    196096,
    208267,
    220993,
    234291,
    248176,
    262666,
    277779,
    293531,
    309941,
    327026,
    344804,
    363294,
    382515,
    402486,
    423225,
    444752,
    467086,
    490248,
    514258,
    539136,
    564902,
    591577,
    619183,
    647740,
    677271,
    707797,
    739339,
    771921,
    805565,
    840294,
    876129,
    913096,
    951217,
    990515,
    1031016,
    1072742,
    1115719,
    1159971,
    1205523,
    1303823,
    1340870,
    1377638,
    1414198,
    1450612,
    1486933,
    1523205,
    1559465,
    1595747,
    1632078,
    1668484,
    1704986,
    1741603,
    1778352,
    1815248,
    1852303,
    1889530,
    1926939,
    1964540,
    2002341,
    2040350,
    2078573,
    2117018,
    2155689,
    2194593,
    2233733,
    2273114,
    2312741,
    2352616,
    2392743,
    2433126,
    2473766,
    2514668,
    2555833,
    2597263,
    2638961,
    2680929,
    2723168,
    2765680,
    2808467,
    2851530,
    2894871,
    2938490,
    2982390,
    3026572,
    3071035,
    3115782,
    3160814,
    3206130,
    3251733,
    3297623,
    3343800,
    3390266,
    3437020,
    3484065,
    3531399,
    3579025,
    3626942,
    3675151,
    3723652,
    3772447,
    3821534,
    3870916,
    3920591,
    3970561,
    4020826,
    4071386,
    4122242,
    4173394,
    4224842,
    4276586,
    4328628,
    4380966,
    4433601,
    4486535,
    4539766,
    4593295,
    4647122,
    4701247,
    4755672,
    4810395,
    4865417,
    4920738,
    4976359,
    5032279,
    5088498,
    5145018,
    5201838,
    5258957,
    5316377,
    5374097,
    5432118,
    5490440,
    5549062,
    5607985,
    5667208,
    5726733,
    5786560,
    5846687,
    5907116,
    5967846,
    6028877,
    6090211,
    6151846,
    6213782,
    6276021,
    6338562,
    6401404,
    6464549,
    6527996,
    6591745,
    6655796,
    6720149,
    6784805,
    6849764,
    6915025,
    6980588,
    7046455,
    7112623,
    7179095,
    7245869,
    7312946,
    7380326,
    7448009,
    7515995,
    7584284,
    7652876,
    7721771,
    7790969,
    7860470,
    7930275,
    8000383,
    8070794,
    8141508,
    8212526,
    8283847,
    8355471,
    8427399,
    8499630,
    8572165,
    8645003,
    8718145,
    8791590,
    8865339,
    8939392,
    9013748,
    9088408,
    9163372,
    9238640,
    9314211,
    9390086,
    9466265, -- 250
}

local function approximate(lv, a,b,c,d,e)
    local v = a
    v = v * lv + b
    v = v * lv + c
    v = v * lv + d
    v = v * lv + e
    return math.floor(v/1000)
end

local function distance(a,b,c,d,e)
    local s = 0
    for i = 1, 98 do
        local v = approximate(i, a,b,c,d,e)
        s = s + math.pow(v - exp[i], 2)
    end
    return s
end

local mindist = 1e30
for a = 9,11 do
    for b = 280, 300 do
        for c = 1000, 1100 do
            for d = 1000, 1300 do
                for e = 300, 700 do
                    local dist = distance(a,b,c,d,e)
                    if dist < mindist then
                        print(string.format('%4i %4i %4i %4i %4i %s',
                            a,b,c,d,e, dist))
                        mindist = dist
                    end
                end
            end
        end
    end
end