The classes i usually see in PvP are High Priest, Whitesmith, Sinx, sometimes hunter/sniper but those first 3 are definitive the ones you usually see most.
Mass battles are usually pretty chaotic, i also think it depends on the class you will be playing (ranged, non ranged etc) seeing Sniper is usually the most popular class for ToH you could develop a strategy for that. Most players are also in PvM build instead of PvP, i wouldn't say its necessary to get a PvP build for mass.
Most people use all kinds of strategies for PvP, status effects are pretty common, and depending on the class you play against, some play by the strategy of gaining enough time for a tie (run away, SW/Pheuma, hide etc), others go head on for a win.
Hope this helps a bit