...is it really that much to be first? I used to be first quite a few times long long ago, doesn't matter...
Anyways, Mightyena is a fairly interesting card. 90 HP is average for a Stage 1, +20 Weakness to Fighting is ok since Fighting isn't huge, -20 Resistance to Psychic is nice, and 1 Energy for a Retreat Cost is good. Bite On is the main reason for using Mightyena; it deals a solid 30 damage for 1 Dark Energy, and if the defending Pokemon isn't evolved, they cannot retreat, attack, use Powers, etc. during their next turn. This single-handedly counters AMU's biggest threat, Mewtwo Lv X, making it a worthy option in AMU decks. Mightyena himself also counters AMU itself and the (soon to be) new SP Pokemon from Galactic Conquest. Harass is Mightyena's other attack, dealing 10x the number of Pokemon you have in play for 2 Colorless Energy; generally, you'll never bother with this attack, unless you've had spare energy to power this attack up or the opposing Pokemon is in KO range.
Modified - 7/10 (Solid counter to some of the more popular cards, but is a tech at best, not something to build a deck around)
Limited - 10/10 (Great card, can stall nicely with Bite On since getting Stage 1s or 2s in Limited is hard)
Unlimited - 1.5/10 (No real use except against the odd haymaker)