creepers should be able to break the raised areas to add challenge and there are other was to get up to the other area e.g. enderpearls. I also stated that you can use performance enhancing mods. the arena is also specifically designed to try to keep the lag down I myself can't handle many mobs on screen at once but there not all rendered at once because of the size and there are no other blocks for a 50 by 50 chunk area. all this leaves me at 40+ fps unless there is a lot of stuff near me.